The document describes a study that experimentally removed wood mice from one site to test for competition with bank voles. It provides background information on the ecology of both species. Wood mice and bank voles were trapped from two sites over four sessions. Population estimates, reproduction, body mass, age structure and spatial patterns were compared between sites. After removal, the wood mouse population recovered within three weeks while bank vole numbers increased later. There were some differences between sites but no clear evidence that removing wood mice benefited bank voles.

1. A Removal Experiment on a Population
of Wood mice and Bank voles
By
Aidan P. Louth
Submitted for B.Sc. (Hons) Degree
March 2015
Department of Zoology
National University of Ireland, Galway

2. 2
Abstract
Introduction .......................................................................................................................................4
The wood mouse.................................................................................................................................4
The bank vole......................................................................................................................................5
1.1 Distribution and population density.............................................................................................6
1.2 Feeding..........................................................................................................................................7
1.3 Breeding........................................................................................................................................8
1.4 Activity ........................................................................................................................................10
1.5 Home range and Dispersal..........................................................................................................11
1.6 Population Structure and Survival ..............................................................................................13
1.7 Competition ................................................................................................................................14
1.8 Aims and Objectives....................................................................................................................15
Materials & Methods....................................................................................................................16
2.1 Study site.....................................................................................................................................16
2.2 Site A ...........................................................................................................................................17
2.3 Site B ...........................................................................................................................................18
2.4 Materials and Methods...............................................................................................................19
Results................................................................................................................................................21
3.1 Trapping Results..........................................................................................................................21
3.2 Population size............................................................................................................................22
3.3 Age structure...............................................................................................................................24
3.4 Reproduction ..............................................................................................................................26
3.5 Mean Body Weight .....................................................................................................................27
3.6 Spatial segregation......................................................................................................................29
Discussion..........................................................................................................................................30
4.1 Conclusion...................................................................................................................................36
References........................................................................................................................................37

3. 3
Abstract
An experimental removal was carried out between the wood mouse Apodemus sylvaticus
(Linnaeus, 1758) and the bank vole Myodes glareolus (Schreber, 1780) to test if there was
interspecific competition present between the species. Four trapping sessions were
conducted between September 2014 and February 2015 in a mixed deciduous woodland on
the outskirts of Galway city. The experiment involved the use of two grids; one as the control
and the other the experimental. A population of wood mice were removed from the
experimental grid after the first trapping session. The populations on each grid where then
monitored without further manipulation for the remaining three trapping periods.
Population estimates, reproductive conditions, body mass, age structure and spatial
segregation where compared between the two sites. The recovery of the wood mouse
population was also monitored.
The wood mouse population recovered within three weeks of the removal. Bank vole density
rose from low numbers to high densities in January. Wood mice maintained relatively high
densities throughout the experiment and exceeded average densities recorded in other
studies. There was a weak negative association between the two species. There was some
evidence of winter breeding in both species. Male wood mice on the experimental grid were
significantly heavier than on the control, the experimental grid maintained higher population
densities than the control for both species. In conclusion there was no evidence of
competitive release on the bank vole from the removal of wood mice.

4. 4
Introduction
The wood mouse
Apodemus sylvaticus, Linnaeus 1978
Figure 1. The wood mouse Apodemus sylvaticus.
The wood mouse can be recognised by its large ears and eyes and long tail (Fig 1). Male wood
mice are generally large than females. The length from nose to tip of tail is about 16-19cm in
adult males and 15-18 in adult females. The average length of head and body combined is
93mm for males and 91mm for females. Average tail lengths 83mm in males and 82mm in
females. The hind feet have five toes and are quite large average length 22mm, the fore feet
have four toes. The hind feet of wood mice are larger than bank voles. Adult males usually
weigh between 20-27g, females 17-24g (Hayden & Harrington 2000). Wood mice on some
offshore islands can weigh considerably more (Fairley et al. 1978).
Wood mice communicate by visual signals which are important in aggressive encounters
along with olfactory and ultrasonic communication. Scent marking is by subcaudal scent
glands in normal movement and is used to indicate sex and sexual state (Corbet & Harris
1991).

5. 5
The bank vole
Myodes glareoulus Schreber 1780
Figure 2. The bank vole M. glareoulus
The bank vole is small rodent and can be distinguished from the wood mouse by its blunt nose
and small ears and eyes. The tail of the bank vole is much shorter than that of the wood
mouse and is covered in fur (Fig. 2). Total body length from nose to tip of the tail is about
15cm. Tail length is about 5cm. Males are heavier than females with adult males weighing
on average 25g and females 22g (Hayden & Harrington 2000). There are four toes on the fore
feet and five on the hind feet. Hind foot length in bank voles is considerably smaller than
wood mice averaging 16.7mm.
They communicate by ultrasonic squeaks, also teeth chattering sound in aggressive
encounters. They also use scent glands to mark their territories. (Hayden & Harrington 2000).

6. 6
1.1 Distribution and population density
The wood mouse is widespread throughout much of Europe including Iceland and western
Asia, and its range extends from central Scandinavia to Northern Africa. The earliest fossil
record of wood mice found in Ireland is 7600 years old (Hayden & Harrington 2001). The
arrival of the wood mouse to Ireland was shown to be anthropogenic and is thought to have
resulted from multiple accidental introductions from Britain and Scandinavia (Corbet 1961,
Berry 1969). The wood mouse is now ubiquitous in Ireland and is found on many of Ireland’s
offshore Islands (Fairley 1984).
The bank vole has a more northerly distribution than the wood mouse, extending as far north
as Lapland and into western Russia. It is found throughout Europe; in the south it is mainly
montane in Northern Iberia and Italy (Stenseth 1985). The bank vole is a recent introduction
to Ireland and was first discovered in Kerry in 1964 (Fairley 1975). Mitochondrial studies have
shown the expansion of the bank vole is from a small group of founding members, which is
thought to have originated in Germany. It is believed to be associated with construction of
the Shannon Hydro Electric Scheme in the 1920s. Samples of bank voles from the localities
from where the turbines were made in Germany closely match the two Irish bank vole
haplotypes (Stuart et a. 2007). Since its introduction it has colonised all of Munster and can
be found as far north as Knockma in north Galway, and as far east as Kilkenny (McHugh &
Boyle 2010).
Wood mice are highly adaptable and are found in nearly every habitat type. Habitats that are
not favourable are areas with water logged soil, where burrows may become saturated. Mice
prefer habitats with moderate cover and are less dependent on ground cover than bank voles
(Fairley 1972). They are most abundant in woodland, and population densities in these
environments are largely dependent on the tree species and food abundance. Mixed
deciduous woodland is favoured, with good cover at the understory level (Fitzgibbon 1997).
Peak densities of 92 per hectare in yew, 24 in oak, 45 in beech and 20 in pine forests have
been recorded in Ireland (Smal & Fairley 1982).
The preferred habitat of bank vole is mixed deciduous woodland with thick shrub or field layer
and fallen trees which are used as nest sites (Apeldoorn et al. 1992). They can also be found
in grassland, and coniferous forests. The abundance of bank voles is associated with the
seasonal variation of ground cover and will be most abundant in areas providing adequate
cover at particular times of the year (Kikkawa 1964). The herb layer is of particular
importance to bank voles providing both cover from predators and as a food supply
(Fitzgibbon 1997). Densities of 130 per hectare have been recorded in woodland in England
(Newson 1963). Although these densities are rarely reached.

7. 7
In fragmented farm woodland densities are dependent on the amount of connectivity
between isolated woodlots and land use in surrounding fields, woodlots that are well
connected by hedgerows are able to support higher densities of wood mice and bank voles
(Fitzgibbon 1997). Arable land is preferred to pasture. Wood mice will often move onto
arable land from surrounding woodland when crops are at a height where they provide good
cover, usually in late spring or early summer and they will remain in the fields and breed until
the harvest, after which they move back into the woodland or hedgerows to overwinter
(Kikkawa 1964, Fitzgibbon 1997). Bank voles in these habitats tend to remain near the
periphery of cover and rarely shift their home range into fields (Fitzgibbon 1997). Unlike in
Britain where the field vole Microtus agrestis is the dominant species on grassland, the wood
mice in the absence of this species will occupy grassland in Ireland. Though in this habitat
they tend to be restricted to areas with good cover (Fairley 1972). Wood mice have been
recorded at low densities on blanket bogs and on the karst landscape of the Burren. (Gallagher
& Fairley 1979)
In the urban environment wood mice and bank voles make use of areas with cover such as
woodland, parkland, allotments, cemeteries, gardens and scrub (Dickman & Doncaster 1987).
The population densities in these habitat patches can be associated with the level of human
disturbance. A study in Terryland Forest Park in Galway city centre found relatively high
numbers of wood mice in an undeveloped area of the park and bank voles in a recently
planted area (Haigh & Lawton 1997). Good quality urban environments can support relatively
high densities of wood mice (57 ha) and bank voles (16 ha) particularly scrub, orchard and
woodland patches (Dickman & Doncaster 1989). High densities in these environments are a
consequence of inhibited dispersal and reduced predation. Bank voles tend to be less
abundant than wood mice in urban environments due to their preference for thick ground
cover (Dickman & Doncaster 1987, 1989).
1.2 Feeding
The wood mouse is an opportunistic omnivore, its diet depends on what is seasonally
available (Hayden & Harrington, 2000). Seeds, green plant material, fruits, fungi and animal
matter are all consumed (Corbet & Harris, 1991). Seeds are of particular importance to the
wood mouse, contributing up to 70% of their diet (Fairley 1984), this figure can raise to as
high as 88% between July and February when seeds become more readily available (Watts,
1968). During spring, green plant material can make a substantial contribution, particularly if
there was a poor seed crop the previous year. This consists mainly of shoots from
dicotyledonous plants (Watts 1968). Fairley (1967) and Miller (1954) showed that animal
matter is eaten throughout the year making a 30% contribution to their overall diet (Hansson
1971). With the arrival of spring and early summer animal matter can make up a significant

8. 8
proportion of their diet, up to 88% in the absence of seeds during this period. Invertebrates
eaten by the wood mouse consists mainly of insect larvae, earthworms and snails (Watts
1968). Fruit is consumed when it is available during the autumn months, mainly hawthorn
berries, elder berries, blackberries and other fruiting bush and tree species. Miller (1954) and
Smal (1980) observed that the mice readily ate the seeds of these fruits and tended to discard
the flesh. On arable land sown, shed and harvested grain make up the bulk of the diet along
with arthropods in spring and weed seeds and grass flowers in summer (Green 1979).
Differences in diet exist between the sexes and different age classes, although much the same
throughout the year males tend to eat more animal matter during the summer months than
females, while females eat more seeds than males. Adults were also found to eat more
animal matter than sub-adults. Juveniles eat less seeds than sub-adults and more fruit, gall
and flowers (Watts 1968).
The bank vole is largely vegetarian in its feeding habit and in contrast to the wood mouse,
seeds only contribute on average 10% of their diet (Watts 1968). In a deciduous woodland
the main bulk of their diet is comprised of green leaf material which is of most importance
during spring and summer. Voles show a preference for the leaves of woody plants such as
oak and sycamore over readily available herbaceous plants. Species of fungi are eaten
throughout the year, the amount of which increases from May to September when the
fruiting bodies emerge (Watts 1968). Although invertebrates are not as important to the bank
vole as to the wood mouse there is evidence that invertebrate larvae and earthworms are
eaten with a peak in May (Watts 1968 and Hansson 1979). Fruit becomes a major part of
their diet in autumn and early winter, unlike the wood mouse the fruits of bushes and trees
are eaten and any seeds with hard tests are discarded (Watts 1968 and Miller 1954).
Dead leaf matter makes up a substantial portion of their diet, particularly during winter and
early spring (Watts 1968). Although this may be eaten for the fungi that is found on the leaves
rather than the leaves themselves as fungi is a common food for voles (Williams and Finney
1964). Tree bark is also eaten during times of increased competition for food, the dead outer
layer of bark is removed and the living bark underneath is consumed (Corbet & Harris, 1991).
There is no difference in the diet of male and female bank voles while young bank voles eat
less seeds than their adult counterparts (Watts 1968). Different foods become more or less
important depending on the habitat type, in coniferous forests berries and fungi are more
important than seeds and insects, and a high proportion of lichens are eaten (Hansson 1979).
1.3 Breeding
Wood mice are typically seasonal breeders with the main breeding activity occurring from
April to October (Tanton 1969). The breeding season reaches a peak in July (Gurnell 1978).

9. 9
Males become fecund earlier than females (Miller 1958). Males are considered fecund when
the testes descend and become scrotal (Fairley 1984). In females in breeding condition the
vagina is perforate or it may have a mucous plug during pregnancy (Smyth 1966). Males are
found to reach breeding status at 15g in weight, females can reach sexual maturity at 12g
(Miller 1958). The gestation period for the wood mouse is between 19-20 days. The female
can become pregnant again shortly after giving birth. The gestation periods for these
pregnancies are extended due to delayed implantation.
Litter size in the wild can be anywhere from two to 11, but the mean range is five to six per
litter (Corbet & Harris 1991). Litter sizes tend to be reduced in winter (Smyth 1966). The
young are born blind and naked at a weight of between one and two grams. The young are
weaned and leave the nest after 18 days at a weight of six grams (Corbet & Southern 1977).
Juvenile males born early in the summer can mature rapidly and breed in the same season.
Those juveniles born late in the season postpone puberty until the following breeding season
(Randolph 1973).
Mice can have anywhere from one to six litters in a breeding season. The last pregnancies
usually occur in October or November however, winter breeding is not uncommon (Smyth
1966). In one study the population of mice in January was 33% juveniles and 25% adults in
breeding condition, this would suggest that breeding continued throughout the winter
(Gurnell 1978). Winter breeding has been associated with good seed crop from the previous
autumn (Smyth 1966). Breeding may also occur without abundant food, when population
densities are low (Fairley & Comerton 1972).
Like wood mice bank voles tend to be seasonal breeders. The time of increased breeding
activity is between April and October (Corbet & Southern 1977). Males and females become
fecund in March. Both male and female voles at a weight of 12 grams are considered sub
adult and are capable of breeding (Tanton1969). Gestation lasts 17 to 18 days but is
prolonged during lactation. The average litter size is four, but can be as many as seven and
pregnancies reach a peak during May and July (Corbet & Southern 1977). Litter size decreases
with increased population density and the associated reduction in food availability (Bondrup-
Neilson 1986).
Young weigh two grams when born and are weaned after 18 days (Corbet & Southern 1977).
The weaning success of females is strongly density-dependent (Ylönen et al. 1999). Voles
which are born early in the season may reach maturity and be recruited into the breeding
population, those born late in the breeding season delay puberty and overwinter as immature
sub-adults. Sexual maturation of young female voles can be suppressed by the presence of
mature female voles (Bondrup-Neilson 1986).
The presence of mature males can also suppress the maturation of immature males
(Gustafsson 1985). Breeding can occur through the winter if there is an adequate supply of
food although litter sizes tend to be smaller (Smyth 1966). The breeding season can be

10. 10
induced earlier in voles if there is an adequate supply of quality food and a low population
density (Eccard & Ylönen2001).
1.4 Activity
Wood mice are keen, alert and will leap out of danger when startled. Movement is by walking
or “scurrying” across open ground, slow deliberate movements are used when investigating
(Corbet & Southern 1977). They are alert while feeding and an unfamiliar noise will cause a
sudden pause and intense listening (Kikkawa 1964). They are accomplished climbers and
spend up to 20% of their time foraging above ground level. Abandoned bird’s nests are often
used as feeding refuges (Buesching et al. 2008). Wood mice are mainly nocturnal (Brown
1954) and have different activity periods depending on the season. In spring and autumn
mice exhibit a bimodal activity rhythm. Peaks of activity occur 2-4 hours after sunset and 3-
4 hours before sunrise. In the summer when nights are shorter and in the height of the
breeding season there is only one active period, mice rarely return to the nest during this
period.
It has been found that pregnant or lactating females have a diurnal activity period during the
summer around midday but males remain below ground. During winter mice make multiple
excursions from the nest for periods of up to 2 hours (Wolton 1983). (Jensen 1993) showed
that Apodemus have temporal changes in food preference with foods higher in sugar sought
out early in the first active periods. Factors governing activity habits are temperature, rainfall
and moonlight intensity. Moonlight has the greatest affect in winter when there is less
ground cover (Brown 1956). Daylight hours are spent below ground in burrow systems that
can be up to 3m deep (Jennings 1975). Wood mice cache food to survive the hours spent
below ground during daylight hours of the summer and harsh weather conditions of the
winter (Jennings 1975).
Bank voles usually run in a ‘hurried’ fashion (Corbet & Southern 1977). They are reluctant to
cross open ground and prefer to travel through areas of thick undergrowth (Kikkawa1964).
Like wood mice, bank voles are well adapted for climbing, and forage in the understory of
trees and bushes provided there is good cover. A peak in this arboreality behaviour occurs in
autumn when trees and shrubs come into fruit (Buesching et al. 2008). Bank voles are
predominantly diurnal with two peaks of activity at dawn and dusk, with some nocturnal
activity occurring during summer (Greenwood 1978). They are active at low temperatures
(Kikkawa 1964) and heavy rainfall will delay their appearance above ground (Brown 1954).
Voles will make runways through cover to get to feeding sites and will often return to the nest
using the same route (Greenwood 1978). They are active burrowers and nest underground
or in tree trunks (Corbet & Harris 1991). Voles cache food at the end of the breeding season,
particularly in autumn if densities are high (Mappes 1998).

11. 11
1.5 Home range and Dispersal
The possession of a home range is a vital need for small mammals, and can be defined as
“that area traversed by an individual in its normal activities of food gathering, mating and
caring for young” (Burt 1943). Wood mice show high variability in the size of home ranges
depending on age, sex, season as well as habitat and food availability (Kikkawa 1964). Male
home range are usually larger than females, 2250m2 for males – 1817m2 for females in a
deciduous woodland (Crawley 1969) although these figures are subject to the method used
in calculating range size (Kikkawa 1964, Korn 1986, Watts 1970). Larger dominant males tend
to have larger home ranges and thus access to more females (Korn 1986). Female home range
decrease in size with the onset of the breeding season (Korn 1986). In contrast males show a
larger shift in home range and a dramatic increase in size when females reach breeding
condition in April (Randolph 1977).
There is a change in spatial associations in the breeding season from predominantly
monosexual aggregations in winter to bisexual aggregations in summer. Wood mice exhibit a
high degree of overlap between male and female home ranges during the breeding season
with increased spacing between males (Randolph 1977). This spatial intolerance is reduced
in winter and mice will often nest in communal burrows (Wolton 1983). Juveniles which are
born early in the summer can attain home range sizes comparable with adult mice within two
months, while those that overwinter as immature adults will maintain small ranges until the
following spring (Randolph 1977). There is no evidence of territoriality in wood mice however
females will aggressively defend the nest site if lactating (Corbet & Harris 1991). Once a home
range is established by adult mice there is a tendency to remain within it or its vicinity for life
(Brown 1956).
Wood mice exhibit seasonal variations in their movements. Greater movements are seen just
prior to and during the breeding season, males are generally more mobile than females
(Crawley 1969). They will often change nest sites within their home ranges and may use old
nest sites as food stores (Kikkawa 1964). Crawley (1969) found that 80% of males and 90% of
females rarely made moves beyond their home range lengths once a home range was
established. On occasion mice may leave their home range for a short period of time, only to
return again. These occasional sorties may be for food exploitation (Kikkawa 1964).
Permanent moves greater than 130m can be considered dispersals in wood mice (Watts
1970). This usually occurs in the breeding season when there is intensified spacial behaviour
between sexes, established and un-established individuals (Randolph 1977). Resident mice
will cause immigrants to disperse, and as a result juveniles and sub-adults are most likely to
make dispersal moves which can be greater than 1km (Dickman & Doncaster 1989).
These dispersal movements tend to be from areas of high population densities to less
populated areas (Dickman & Doncaster 1989). Early breeders may disperse before the

12. 12
breeding season starts and travel long distances in search of mates (Kikkawa 1964). Dispersal
in wood mice is likely driven by aggressive interactions with the onset of the breeding season
(Montgomery & Gurnell 1985).
Female bank voles home range size is largely dependent on resource availability and breeding
condition, the home range includes the foraging range and a smaller breeding territory.
Breeding females are territorial and will not tolerate other females in their breeding territory
(Ylönen et al. 1997). Home range size in females will be reduced if there is increased food
quantity and quality (Bondrup-Neilson & Karlsson 1985). During the reproductive cycle
female home range or the foraging area will decrease in size, while the breeding territory will
be at its largest at the time of parturition and spacing between females increases (Ylönen et
al. 1997). Whereas female home range size is based on available resources, male home range
size is dependent on the distribution of females (Ims 1987).
Studies have shown that males have larger home range than females and various figures have
been published on the estimated size .19 ha for males .13 ha for females (Crawley 1969), .22
ha for males and .112 ha for females (Kikkawa 1964). The amount of overlap between female
ranges is dependent on the distribution of resources; if there is an even distribution of food
then there may be some degree of overlap. If food is unevenly distributed then the overlap is
reduced. Alternatively males show extensive overlap of their territories and these will usually
encompass several female ranges (Bondrup-Neilson & Karlsson 1985). If females are evenly
distributed in a population then males reduce territorial behaviour and will wander in search
of mates. On the other hand if females are patchily distributed males will actively defend
territories that contain females (Ims 1987).
Male bank voles have a social hierarchy with dominant and subordinate individuals
(Rozenfield et al. 1987). Territoriality is reduced in nonbreeding populations in winter.
Overwintering populations may form small exclusive groups of two-three females and a male,
and nest in communal burrows (Ylönen & Viitala 1991). Upon reaching sexual maturity males
expand their ranges at the beginning of the breeding season and female ranges contract when
breeding (Korn 1986). Unlike wood mice female bank voles require a territory in order to
mature sexually and breed (Bondrup-Neilson & Ims 1986).
Bank voles are more restricted in their movements than wood mice due to their preference
for good ground cover (Kikkawa 1964). They are a more sedentary species than wood mice,
moving shorter distances in their dispersal moves and roaming over smaller home ranges.
Dispersal in bank voles is not as seasonal as wood mice and continues throughout the year,
with peak dispersal occurring in spring and summer and males showing greater mobility than
females (Crawley 1969). Female bank voles with established territories and in breeding
condition rarely disperse. The majority of dispersal moves are made by juveniles and sexually
immature sub-adults (Bondrup-Neilson & Karlsson 1985). Juveniles disperse from the natal
site prior to puberty at a weight of 12g and dispersal distances tend to be small (Watts 1970).

13. 13
Bank voles exhibit strong site tenacity with the area of birth (Ylönen & Viitala 1991). Distances
travelled greater than 110m are considered dispersals for bank voles (Watts 1970).
1.6 Population Structure and Survival
Wood mice populations show seasonal fluctuations in size and age structure. Winter
populations are high consisting of mature adults and overwintering immature sub-adults.
Numbers decline in spring with the onset of the breeding season and low numbers are
maintained throughout the summer. Populations increase in numbers in autumn with the
cessation of the breeding season until winter highs are reached (Watts 1969). Overwinter
survival is associated with food availability and good mast crop (Watts 1969). The spring
decline is brought about by increased aggressive interactions between male mice and the
establishment of exclusive breeding areas by females, adult survival during this time is poor
(Gurnell 1978 and Flowerdew 1974). Established mice hinder the settlement of immigrants
and juveniles during the breeding season which results in poor survival of these age classes
during spring and summer. The majority of young born in spring and summer disperse or die.
Laboratory experiments have shown that resident mice will sometimes isolate or kill strange
juveniles (Flowerdew 1974). The autumn increase in numbers is a result of better survival of
juveniles and sub-adults due to the reduction in intra-specific strife with the end of the
breeding season. Overwintered mice rarely survive to autumn, and autumn populations
consist mainly of young of the year (Gurnell 1978). Removal of adult male mice will bring
forward the autumn increase and improve recruitment of young age classes (Flowerdew
1974). Population numbers in wood mice are regulated by these social interactions during
the breeding season and by food availability over winter (Watts 1969).
Bank voles also exhibit seasonal fluctuations in their population densities. These can vary
from year to year. Most annual cycles show population highs in autumn and winter, followed
by declines in spring when breeding commences. Densities begin to rise again during summer
and reach their peak in autumn or early winter (Newson 1963, Tanton 1969). Populations can
increase in size from winter to spring and early summer after winter breeding (Newson 1963).
Young born in the latter half of the year tend to overwinter as immature sub-adults, these
then grow rapidly in spring with the onset of sexual maturity (Corbet & Harris 1991). The
survival rate for first born young of the year is poor and summer survival tends to be low
however, survival improves in autumn and winter (Smyth 1968).
The decline in numbers in spring is associated with agonistic interactions and territoriality of
the breeding season (Rozenfield et al. 1987). When breeding ceases survival of younger age
classes improves. In northern population voles can have multi-annual cycle fluctuations in
their densities, these are characterised by three year annual cycle followed by low densities
in the fourth year (Hansson & Henttonen 1985). These multi-annual cycles are determined
by intra-population density dependent mechanisms (Zhigalski & Kshnyasev 2000). The life-

14. 14
span of bank voles may be influenced by population density, with shorter life-spans found at
high densities (Corbet & Harris 1991).
1.7 Competition
Competition among species strongly affects many animal communities and interspecific
competition is widespread among rodents. Grant (1972) suggested that the main
interspecific competition among rodents is for space and he identified three states of
coexistence between rodent species: I. Coexistence with competitive interaction, II.
Coexistence with competitive interaction where the outcome of which can have predictable
or unpredictable outcomes and III. Coexistence without competitive interaction. He
concluded that the coming together of two species that were previously allopatric may
progress through the three states where eventually selection would lead to ‘peaceful
coexistence’ over a wide area of sympatry. Alternatively the evolutionary pathway would
lead to contiguous allopatry with only small extreme areas of sympatry. Selection will always
tend to minimize the overlap in diet, spatial and temporal activity patterns. The role of
interspecific competition and the effect it has on small mammal assemblages is still uncertain.
Removal experiments can give evidence on whether competition exists between two or more
species (Fasola & Canova 2000). Whereby the removal of one species can lead to competitive
release in the other if competition is a factor in their coexistence. Montgomery (1981)
performed a removal experiment on A. flavicollis and A. sylvaticus and recorded no evidence
for competition between the species despite the fact that spatial segregation exists due to
aggression. He concluded that differences in dynamics and population regulation in these
species led to their stable coexistence. Gliwicz (1981) experimentally removed the yellow-
necked mouse A. flavicollis and the bank vole M. glareolus from a population containing the
striped field mouse A. agrarius, which is comparable in size and dietary preferences to the
wood mouse and found that the removal of these two species led to competitive release in
the striped field mouse. She hypothesised that the striped field mouse competed with the
yellow necked mouse for food and with the bank vole for space.
Differences in diet (Watts 1968, Hansson 1978), activity rhythms (Greenwood 1978) and
micro-habit preference (Apeldoorn et al. 1992) result in different realized niches for the wood
mouse and the bank vole (Fasola & Canova 2000). This segregation of niches indicate that
some competitive interactions exist. Greenwood (1978) suggested that bank voles are more
nocturnal in the absence of the wood mouse, which suggests some existence of interference
competition. No distinct behavioural hierarchy exists when the two species interact however,
whichever animal is heavier usually induces a flight response in the other (Ambin & Bauchau
1989). Fasola & Canova’s (2000) removal experiment revealed asymmetrical competition
between the bank vole and the wood mouse. They found that the removal of the bank vole
had no effect on the wood mouse, but the removal of the wood mouse led to an increase in
population density of the bank vole.

15. 15
1.8 Aims and Objectives
Most evidence for interspecific competition between two sympatric species falls into two
categories; two sympatric species show an inverse in numerical relationship or the two
species exhibit an inverse spatial relationship (Grant 1972). The most effective to way gather
this evidence is to artificially lower the density of one species in the presence of the other
either by exclusion or removal (Fasola & Canova 2000). Bank voles and wood mice are the
co-dominant rodents across much of Europe’s woodlands. Although they may compete for
food and space the lack of relationship between the abundance of one species and that of the
other suggest that some other factors determine niche segregation (Fasola & Canova 2000).
They concluded that the only way to way to discover the prevalence of competition between
the two species is by removal experiments.
The objective of this study was to investigate if the bank vole exhibited some form of
competitive release in terms of; population density, reproductive condition, body mass and
population structure, with the removal of the wood mouse. In doing so, reveal if the bank
vole is suppressed by the wood mouse.
 Aim 1: To examine the effect the removal of wood mice would have on a bank vole
population.
 Aim 2: To examine the effect the removal of wood mice would have on a wood
mouse population.
 Aim 3: To assess the populations of both wood mice and bank voles and to compare
the findings with those of previous studies.

16. 16
Materials & Methods
2.1 Study site
Field work was carried out in Unclín Wood which is located 3km east of Galway City centre.
This woodland used to be a part of the Merlin Park estate but is now cut off from the Merlin
Park woodland by the R338 Dublin to Galway road. The land was originally part of an estate
owned by Charles Blake who built Merlin Park house in the first decade of the 19th century
however, the house no longer exists. The land was inherited by the Waithman family in 1876
who subsequently sold the land to the state in 1945 so a tuberculosis sanatorium could be
built which is now the site of Merlin Park hospital. Unclín Wood is now in the possession of
Galway city Council (Browne & Fuller, 2009).
The woodland is enclosed on all sides by roads, the R338 to the North and the old Dublin
road to the South (Fig 1). It is a broadleaved deciduous woodland approximately 4ha in
Figure 3. Aerial picture of the woodland. The wood is enscribed by the yellow line. Grid A is represented by
the blue box, grid B is represented by the red box. Each are 40m x 20m. Photo courtesy of Google Earth.

17. 17
area. The main tree species found here are Beech (Fagus sylvatica), Hazel (Corylus
avellana), Hawthorn (Crataegus monogyna), Holly (Ilex aquifolium), Sycamore (Acer
pseudoplatanus) and the Horse chestnut (Aeusculus hippocastanum), Beech and Hazel are
the dominant species. The wood is surrounded by pasture farmland to the south and west,
Roscam housing estate to the east and Merlin Park hospital to the North.
2.2 Site A
Site 1 was located in the western area of the woodland (Fig 1) at a narrow part of the wood.
This was the location for the control trapping grid. The canopy layer (>5m+) was dominated
by large mature beech trees (Fagus syvatica) with a canopy covering of 75% during the first
two trapping sessions, this was reduced with the onset of winter, there was no herb layer
beneath these. The shrub layer (1-5m) was a mix of hazel (Corylus avellana), hawthorn
(Crataegus monogyna) and holly (Ilex aquifolium) in dense numbers, again with a covering of
75% until the onset of winter. The herb layer (0.15-1m) was patchily distributed across the
trapping area and mainly consisted of bramble (Rubus fruiticosus), bracken (Pteridium
aquilinum) and ivy (Hedera helix). The ivy provided some cover on the shrub layer by climbing
larger trees. The ground layer (0-0.15m) was mainly Hedera, leaf litter and logs, the amount
of cover at this level was heterogenous with areas of sparse cover in places with only a thin
layer of leaf litter on the ground (See right foreground Fig 2).
Figure 4.2: Trapping Grid A

18. 18
2.3 Site B
Site B was located approximately 150m to the east of Site A (Fig1), this was done to reduce
the chance of movement of animals from one trapping site to the other. This was the location
for the experimental trapping grid. The canopy layer was beech with a coverage of
approximately 50%. This site had a dense understory and shrub layer consisting mainly of
hazel and holly trees. The herb layer consisted of ivy and fallen branches. The ground layer
had denser cover than Site A comprised of ivy, sphagnum moss (Sphagnum cuspidatum),
bracken (Pteridium aquilinum), Hart’s tongue (Phyllitis scopendrium) leaf litter and tree falls
(Fig 3). The ground at this site was uneven and had large boulders scattered throughout the
site.
Figure 2.3: Trapping Grid B

19. 19
2.4 Materials and Methods
Four trapping sessions were carried out over the course of this study: 29th September-2nd
October, 28th -31st October, 16th – 19st January and 13th-16th February. Each session comprised
one day of setting traps and three days and nights of trapping.
The experimental design included a removal grid (hereafter referred to as Grid B) and a
control grid (hereafter referred to as grid A). Prior to removal the animals were captured
marked and released 29th September-2nd October. The removal period 4th-7th October
involved trapping on Grid B and removing any A. sylvaticus that were captured. These were
used in a separate study. Post-removal (28th -31st October, 16th – 19st January and 13th-16th
February) the populations were monitored without further manipulation.
The trap used in this study was the Longworth live trap which was developed by (Chitty &
Kempson, 1949). The trap consists of a tunnel, treadle, trap door and a nest box. Once an
animal steps on the treadle the trap door closes and the animal is contained in the trap. The
treadle sensitivity can be adjusted to respond to different weights. 60 traps were used in each
session, 30 traps for each trapping grid. Each grid had 90 trap-nights per session. Grids were
approximately 20m x 40m covering an area of 800m2 area with a total of 15 trap points per
grid, each grid had three rows 10m apart. There were five trap points along each row. Each
trap point was approximately 10m from its neighbour as this has been found to be the ideal
distance to maximise the chance of trapping Apodemus and Myodes in woodland (Kikkawa,
1964). Two traps were set at each trap point, this was done so that if a trap was occupied
another trap was available to a visiting animal, this reduced competition for traps.
Each trap was numbered so its position could be known in the field. Meadow hay was placed
in the nest box of each trap as nesting material, to provide insulation and reduce mortality in
cold weather (Manville, 1949). The traps were baited with rolled oats and peanuts which are
suitable bait for mice and voles (Manville, 1949, Gurnell & Flowerdew, 1994). Traps were
placed together using any natural cover that was in close proximity to the trapping point.
Wood mice and bank voles tend to avoid open spaces (Gurnell & Flowerdew, 1994) so placing
the traps in cover gives a better chance of trap success. Once the traps were in position they
were covered with surrounding vegetation in order to provide an extra layer insulation for
the animal and to camouflage the traps from the animals themselves and any people passing
by (Fig 2.1). The traps were left in position for approximately 24hr. The same trap locations
were used for each trapping session.
On trapping days each trap was checked and the condition recorded. If the trap was closed
it was broken open into a clear plastic bag and the species was recorded. The animal was
weighed to the nearest 0.5 of a gram. A callipers was used to measure the hind foot length
from the tip of the toe to the ankle to the nearest 0.1 of a mm. A small patch of guard hair
was removed from the rump of the animal in order to mark it. The hair was clipped from
different parts of the rump on each trapping session so that if an animal was subsequently
caught in a following trapping session it would be recognised from which session it was caught
previously. The animal was then released at the point of capture. The trap were cleaned

20. 20
out and fresh nest material and bait was supplied. The trap was then reset and placed in the
same position. The same process was carried out for the next two days. The sex of the animal
and its breeding condition was recorded. Males were judged to be reproductive if the testes
were scrotal, females were deemed reproductive if they had clear patches around their teats
or the vagina was perforate. Animals were classified into age-classes depending on their
weight and reproductive status (Tanton 1969, Watts 1970, Miller 1958). For A. sylvaticus non-
reproductive animals (males ≤14.5g, females ≤11.5g) were classed as juvenile, potential
breeders as sub-adult (males 15-20.5g, females 12-19.5g) and any individuals over this weight
were considered adult or experienced breeders. For M. glareolus individuals less than 12g
were deemed juvenile, potential breeders (males 12.5-21.5g, females 12.5-19g) any and any
individual over these weights, adult. On each trapping session the weather conditions and
moon cycle were recorded for each day of trapping.
The Lincoln Index was used for population estimation. These were calculated by taking the
new animals on the first two days of trapping as the marked animals, multiplying by the total
catch on day three of trapping and dividing by the recaptures on day three.
The spatial relationship between A. sylvaticus and M. glareolus was examined by presence
and absence analysis. This method depends on the frequency of capturing both species, at a
single point, during a 24 hour period. At each point the catch may be A. sylvaticus only, M.
glareolus only (regardless of whether more than one conspecific was caught), both species
together or neither. The data was collated for each trapping period of three days and used
to measure Cole’s coefficient of association (Cole 1949).
Figure 2.4 Example of a trapping station setup with two
Longworth traps side by side at the base of a tree.

21. 21
Results
3.1 Trapping Results
Table 3.1 Total number of captures of A. sylvaticus, M. glareolus, S. minutus and closed empty traps. Figures
in parentheses are total recaptures. * denotes recaptures from the previous trapping period.
Trapping
session
A. sylvaticus M. glareolus S.
minutus
Number
closed but
empty
% traps
closed per
90 trap
nights
Grid A Sep 7 (1) 4 (2) 3 4 20%
Oct 40 (18)1* 3 (1) 1 7 56.7%
Jan 37 (11)2* 19 (6) 1 3 66.7%
Feb 30 (12)7* 28 (12)1* 0 0 64.4%
Total 114 (42) 50 (21) 4 14 51.7%
Grid B Sep 23 (4) 0 (0) 1 7 34.4%
Oct 24 (8) 1 (0) 0 4 32.2%
Jan 25 (4) 26 (7) 1 4 62.2%
Feb 20 (3)3* 35 (13)4* 0 9 71.1%
Total 92 (19) 62 (20) 2 24 50%
Three species were caught during the course of this study: A. sylvaticus, M. glareolus and S.
minutus. The captures of bank voles in both grids in September and October were low with
no voles being caught in grid B in September. Five of the six shrews captured were found
dead in the traps. A juvenile mouse died in the trap during the September session another
female mouse was found dead in the trap during the October session. In the January trapping
session five bank voles were found dead in the traps, one of which was a recapture. In
February there was three vole fatalities one of which was a recapture and one recaptured
wood mouse died. The percentages of closed traps for each period are given in (Table 3.1).
Recaptures between subsequent trapping periods was low which indicated a high turn-over
of the population (Table 3.1). Two animals were caught in the same trap on five occasions,
twice with bank voles and three times with wood mice. Consistently more mice than voles
were caught on grid A throughout the study, in contrast apart from September and October
when only one vole was captured on grid B voles were captured in greater numbers than mice
at this site (Table 3.1). Two mice and two voles escaped while being processed during the
study. There was more recaptures of A. sylvaticus on Grid A than Grid B though the difference
was not significant (t= 1.928, df=3, P>0.05)

22. 22
Table 3.2 Sex, Age class and breeding status of removed wood mice. Figures in parentheses represent mice
in breeding condition
A. sylvaticus
Juvenile Sub-adult Adult Total
Male 4 1 8 (8) 13
Female 1 8 (2) 4 (4) 13
(Table 3.2) gives the information on the wood mice that were removed at the start of October
for the parasitology study. Of the six females in breeding condition two were pregnant and
five were lactating. Four males and three females were captured in in the preceding trapping
period.
3.2 Population size
Table 3.3 Population estimate using Minimum Number Alive Method
Trapping session A. sylvaticus M. glareoulus
Grid A Sep 6 2
Oct 22 1
Jan 26 13
Feb 18 16
Total 72 32
Grid B Sep 19 0
Oct 16 1
Jan 21 19
Feb 17 22
Total 73 42
The population was estimated using two methods: the ‘Minimum Number Alive Method’ and
the ‘Lincoln Index’. The Minimum Number Alive Method (Table 3.3) estimates the size of the
population as the number of new individuals caught over the course of each trapping period.
It assumes that all individuals in the population were captured during the trapping session,
for this reason the Minimum Number Alive Method usually underestimates the population
size. This method is suitable when the number of captures are low or when there are no
recaptures, as was the case for bank voles at site B in September and October (Table 3.1). The

23. 23
population of A. sylvaticus at site A showed a steady increase in numbers until January
followed by a decrease in numbers in February. At site B there was a drop in number from
September to October, this was followed by an increase to a high in January, subsequently
there was a reduction in population in February (Table 3).
Figure 3.2: Population estimate of M. glareoulus
using the Lincoln Index
Figure 3.1: Population estimate of A. sylvaticus using
the Lincoln Index
0
10
20
30
40
50
60
Sep Oct Jan Feb
Estimatedpopulation
Grid A Grid B
0
5
10
15
20
25
30
35
Sep Oct Jan Feb
Estimatedpopulation
Grid A Grid B

24. 24
Numbers of M. glareolus were low in September and October in both grids, January showed
a sharp increase in numbers followed by a gradual increase to a February high. The changes
in population density for A. sylvaticus and M. glareoulus using the Lincoln Index are illustrated
in Figures 1 and 2 respectively. The Lincoln Index for A. sylvaticus for grid A shows a steady
increase in numbers from September to a January high with a subsequent fall in density in
February this agrees with the figures given in (Table 3.3). The density of A. sylvaticus in
September for grid B far exceeded that found by the minimum number alive method for the
same period which was likely a result of the low number of recaptures (two) on the third day
of trapping of this period. There followed an increase in population from October until
February, which was in contrast to figures in (Table 3.3). There was no significant difference
in the mean population density between grid A and grid B for A. sylvaticus using a paired
sample t-test (t = -1.583, df=3, P>0.05) for the study.
The Lincoln Index was not applicable to M. glareolus for either grid in September and October
due to low captures (Table 3.1). There was an increase in the estimated population in January
for both grids, grid B showed a greater increase in population, this was followed by a decline
in February (Fig. 3.2), which contrast with figures given in (Table 3.3). There was no significant
difference in the population densities of M. glareolus between grids (t = -1.313, df=3, P>0.05,
paired sample t-test). This suggests that the removal of A. sylvaticus from site B had a
negligible effect on the population density of M. glareolus as the increase in population
occurred during the same period on both grids.
3.3 Age structure
The numbers of each age class trapped are given in Table 4. The majority of populations of
A. sylvaticus were comprised of sub-adults during the course of the study. September was
the only period when the proportion of adult mice was greater than sub-adults, this was the
case on both grids (Fig 3.3). After the removal of mice from grid B in September (Table 3.2),
there was a significant change in the proportion of age classes in October on this grid (χ2 =
9.23, df=2, P<0.01). This was likely a result of the removal of 12 adult mice from the
population. The proportion of adults in Grid A also fell in October, as a result of an increase
in sub-adult captures and not a decrease in adult numbers caught, this change was not
significant. There was no significant difference in the proportion of age classes between Grids
A and B for each trapping period. Age classes trapped on Grid B in September differed
significantly from those in January (χ2 = 9.23, df=2, P<0.01) and February (χ2 = 11.12, df=2,
P<0.01). Age classes on Grid A in September also showed a significant difference with January
(χ2 = 8.331, df=2, P<0.05) and February (χ2 = 7.85, df=2, P<0.05). A small proportion of the

25. 25
population was juvenile for each month, except in January on grid B when no juveniles were
captured.
The proportion of juveniles on grid B in September was greater than that of sub-adults. The
age-class proportions of M. glareolus are illustrated in Figure 4.3. Sub-adults were the most
numerous age class in the population. There was a significant difference in age-class
proportions between January and February on Grid B (χ2 = 8.56, df=2, P<0.05). Five juveniles
were trapped at this grid in January, while only two adults were trapped during this time,
which resulted in the significant difference. Only one other juvenile was trapped at Grid A in
January. There was a significant difference between age class proportions of M. glareolus
and A. sylvaticus in January on Grid B (χ2 = 7.52, df=2, P<0.05) and February on Grid A (χ2 =
6.02, df=2, P<0.05). A greater proportion of adults was recorded for M. glareolus in February
than was found in A. sylvaticus.
0
0.2
0.4
0.6
0.8
1
Sep A Sep B Oct A Oct B Jan A Jan B Feb A Feb B
n=6 n=19 n=22 n=15 n=24 n=21 n=18 n=17
Proportionofthepopulation
Juvenile Sub-adult Adult
Figure 3.3: Proportion of juvenile, sub-adult and adult A. sylvaticus for
each trapping period. A represents grid A, B grid B. n is the sample size.

26. 26
3.4 Reproduction
The numbers of males and females trapped for both species is given in (Table 3.5) along with
percentages of those animals in breeding condition. There was no significant departure from
the sex ratio of 1:1 in A. sylvaticus at site A (χ2 = 0.01, df = 1 P>0.05) or at Site B (χ2 = 1.718,
df = 1 P>0.05) for any trapping period, this was also the case for M. glareolus at site A (χ2 =
1.63, df = 1 P>0.01) and at site B (χ2 = 0.30, df = 1 P>0.01). These values were calculated using
Yate’s correction for continuity for one degree of freedom. There was no significant
departure from the sex ratio between Grids within each month for either species. The sex
ratio for M. glareolus was significantly different between January and February (χ2=7.51 df=1,
P<0.01)
In A. sylvaticus, a greater proportion of males were in breeding condition than females (χ2 =
18.17, df=1, P<0.001). There was a significantly greater proportion of males in breeding
condition on Grid B than on Grid A (χ2 = 3.86, df=1, P<0.05) throughout the study. September
had the highest proportion of females in breeding condition on both Grids with over 50%
(Table 5). The proportion of females in breeding condition in September on Grid A was
significantly greater than those found in January (χ2 = 8.56, df=1 P<0.01) and February (χ2 =
4.28, df=1, P<0.05). This indicated that while some breeding occurred over winter the main
breeding season ended by October. The only significant proportional difference of females in
breeding condition on site B was between September and February (χ2 = 7.1, df=1, P<0.01).
Figure 3.4: Proportion of juvenile, sub-adult and adult M. glareolus for each trapping
period. A represents Grid A, B Grid B. n is the sample size.
0
0.2
0.4
0.6
0.8
1
1.2
n=1 n=0 n=1 n=1 n=13 n=19 n=15 n=21
Sep A Sep B Oct A Oct B Jan A Jan B Feb A Feb B
Proportionofthepopulation
Juvenile Sub-adult Adult

27. 27
Grid B had a higher number of female M. glareolus in breeding condition in February than
Grid A, although it was not significantly greater. There was no statistically significant monthly
change in the percentage of animals in breeding condition for either sex for M. glareolus, with
numbers in breeding condition remaining relatively constant. Significantly more males than
females were in breeding condition throughout the study (χ2 = 9.26, df=1, P<0.01).
Table 3.5: Numbers of male and female A. sylvaticus and M. glareolus trapped and percentage in breeding
condition. * represents when females were lactating.
A. sylvaticus M. glareolus
Trapping
session
Male Female Male Female
n breeding n breeding n breeding n breeding
Grid
A
Sep 4 75% 2 100%* - - 2 100%
Oct 10 30% 12 33% 1 100% - -
Jan 12 42% 12 8% 7 43% 6 17%
Feb 9 56% 9 22% 4 75% 11 9%
Total 35 46% 35 26% 12 58% 19 21%
Grid
B
Sep 6 83% 13 54%* - - - -
Oct 7 71% 8 13%* 1 100% - -
Jan 9 67% 12 17% 11 46% 8 13%
Feb 8 75% 9 0% 6 67% 15 27%
Total 30 73% 42 24% 18 56% 23 22%
3.5 Mean Body Weight
There was a significant difference in mean weight between male and female wood mice (z =
2.23, P<0.05) and bank voles also showed a significant difference in weight between the sexes
(z = 2.18, P<0.05). Wood mice exhibited a significant difference in their hind foot lengths
between the sexes (z = 4.10, P<0.01). Voles had no significant difference in hind foot lengths
between the sexes (z = 1.74). Males on Grid B were significantly heavier than those on Grid
A (t=-2.997, P<0.01) (Fig 3.5). There was no significant difference between grids for females
(t=-.548, P>0.05) (Fig 3.6). There was no significant difference in hind foot length for females
(F7,68 = 0.756, P>0.05), males had a significant difference in hind foot length (F7,57 = 2.56,
P<0.05). The difference was in September Grid B and January Grid B with larger mice found
in January this was found using a Tukey test.

28. 28
Figure 3.5: Mean monthly weight of male A. sylvaticus
Figure 3.6: Mean monthly weight of female A. sylvaticus
12.5
13.5
14.5
15.5
16.5
17.5
18.5
19.5
20.5
21.5
22.5
Sep Oct Jan Feb
Meanweight(g)
Male A Male B
10
12
14
16
18
20
22
Sep Oct Jan Feb
Meanweight(g)
Female A Female B

29. 29
There was no significant difference in weight or hind foot length for M. glareolus (Weight;
males F5,24 =1.77, P>0.05, Females F5,24 =1.46, P>0.05. Hind foot length; male F5,24 = 1.832,
P>0.05, females F4,37 = 1.5, P>0.05). Figures 3.7 illustrates the monthly change in weight M.
glareoulus
3.6 Spatial segregation
Table 3.6: Cole’s coefficient of interspecific association between A. sylvaticus and M. glareoulus. *
represents significance at the P<0.05 level
September October January February
Grid A 0 0 -0.137 -0.430*
Grid B 0 0 -0.246 -0.013
The coefficients were tested for significance from the departure from zero using the Chi-
square test. 0 represents no association, negative figures negative association and positive
figures positive association. There was no association between species in September and
October and negative association in January and February (Table 3.6). The only significant
departure from no association was in February in Grid A when there was a significant negative
association between the two species (χ2 = 6.421, P<0.05).
10
12
14
16
18
20
22
24
Sep Oct Jan Feb
Meanweight(g)
Male Female
Figure 3.7: Mean monthly weight of male and female M. glareolus

30. 30
Discussion
The number of total captures for the wood mouse remained consistent throughout the study
on Grid B. Total captures rose slightly throughout the study until January followed by a slightly
smaller number in February. This trend in number of captures agrees with other studies on
wood mice in deciduous woodland (Miller 1958, Brown 1954). The results from Grid A
differed from B in that there was a low total catch in September followed by an October high
and a reduction through January to February. The smaller number caught in September on
Grid A is likely an underrepresentation of the true number of mice in the area at this time,
given subsequent catches on Grid B. This was likely due to a change in trap response by
individuals.
With an adequate supply of natural occurring food, particularly tree mast and invertebrates,
foraging mice may be less likely to visit baited traps. During winter when food supply
diminishes mice may show increased interest in the traps as competition for food increases
(Tanton 1965, Watts1969). Several large beech trees were shedding their mast on Grid A at
this time and this locally abundant food supply may have been a reason for the low numbers.
Tanton (1965) found that few animals were trapped if rain started before the emergence of
mice at dusk. Heavy rain occurred during the first two days of this trapping period but was
unlikely to be the cause of the low numbers given the numbers trapped on Grid B. Smyth
(1966) found that animals that were removed from a trapping area in one month were almost
completely replaced by immigrants by the next month, this was the case on Grid B after the
removal of mice at the start of October.
Bank vole captures were low over September and October and it is unclear whether this was
due to a low population density in the woods at this time or whether voles had a lack of
interest in the traps (Kikkawa 1964). Watts (1968) has shown that fallen tree leaves make up
a large proportion of vole’s diet at this time of year; this food source may have been more
appealing than baited traps. Following the low in autumn, voles were then caught in
considerable numbers in January and February. Tanton (1969) recorded high numbers in
August followed by a reduction in numbers caught in September and October with a
subsequent increase in November, through to January at Monks Wood. Without data on
November and December in this study it is not possible to ascertain whether voles followed
this pattern. When voles were trapped in numbers it was revealed that Grid B had higher
number of catches. This was probably due to the preference of voles for good ground cover
(Kikkawa 1964), as ground cover in this area was more homogenous than in Grid A. The
removal of wood mice from this grid appeared to have no effect on the trapability of bank
voles, as captures on both grids increased simultaneously.

31. 31
The number of recaptures on Grid A was greater than that on Grid B, this was likely an effect
of grid placement rather individual’s response to traps. Grid A was located at a narrow part
of the wood and movement into and out of the Grid was restricted by the road to the north,
south and to a lesser extent to the west, as roads act as barriers to small mammal movement
(Oxley et al. 1974). These barriers may have restricted the edge effect to the east side of the
trapping grid. Mice that were trapped in this Grid were more likely to have more than one
trap point in their home range which would give better chance of recapture (Crawley 1969).
In contrast Grid B had no such restriction, and because of the small size of the trapping grid
edge effects were inevitable, with mice having a greater chance of only having a small
proportion of their home range in the trapping grid (Crawley1969). This may have been a
reason for the lower number of recaptures on this Grid. Voles showed the same trend in
recaptures for both Grids increasing in number in February.
Kikkawa (1964) found that with increased population, competition for traps increases and as
a result the number of recaptures. Provided there are enough traps available for all trappable
animals in an area, competition for traps will be reduced. Gurnell & Flowerdew (1994)
suggest that no more than 50-60% of traps should be occupied in a single night. The
percentages of occupied traps rose above this limit in January and February on both Grids,
and the occupied traps recorded in February on Grid B were substantially higher. This
indicated that for these two months there was a high level of inter and intraspecific
competition for traps which possibly led to biased results.
Wood mice were caught in greater numbers for every trapping session on Grid A, while on
Grid B captures of bank voles were greater in January and February. Also wood mice captures
declined from January to February while vole captures increased, this change in dominant
numbers may have been biased towards voles, and because of their diurnal activity pattern
they would have had first access to traps (Greenwood 1978). This competition for traps may
have led to an underrepresentation of wood mice during this time of heightened competition.
This bias can be avoided by adding more traps to each trapping station (Gurnell & Flowerdew
1994)
The Lincoln Peterson Index estimates cannot be not be used to give an accurate estimate of
population density for each species, as the actual catchment area of the trapping grid was not
known. Mice and voles have been found to travel less far in winter, approximately 60% of
animals rarely travel distances greater than 30m (Crawley 1969, Watts 1970, Randolph 1977).
It was not possible to measure the home range of these species in this study, so this figure
was used as the measure of half the home range distance, in order to convert estimated
population numbers into density. It is possible that movements greater than this were made
into the trapping Grids. In order to compensate for edge effects when calculating population
density, a boundary strip of half the mean range length is added to area of the grid, (Brown
1954, Watts 1969, Fairley & Comerton 1982, Crawley 1969). Maximum density on grid A was
41.25 per ha which was recorded in January, Grid B had a maximum density of 68.75 per ha
in September (Fig. 3). Smal & Fairley (1984) gave densities of 6-45 per ha in beech woodland

32. 32
in Ireland. Dickman & Doncaster (1989) gave densities as 58 per ha in undisturbed patches of
urban woodland.
The estimated densities in this study are greater than most of the densities found in a 15 year
survey of over 30 British deciduous woodlands (Flowerdew et al. 2004).
The rise in population density through autumn and winter and subsequent reduction in spring
is a typical trend in wood mouse populations (Tanton 1965, Watts 1969). The reduction in
density in spring coincides with the onset of the breeding season when males begin to expand
their territories (Randolph 1977) and thus are less likely to be trapped in a small trapping grid.
This trend was evident on Grid A. Grid B had the highest population density in September
followed by an October low, which was likely as a result of the removal of mice from this area.
Bank vole densities were calculated with a boundary strip of 30m (Crawley 1969), with max
density estimates of 25 per ha and 41.25 per ha for Grids A and B respectively in January.
Bank vole densities reach 30 per ha in suitable habitats (Hayden & Harrington 2000). Dickman
& Doncaster (1989) found similar densities in urban woodland, although densities on Grid B
were higher than what they recorded. These high densities for both species are probably due
to the restriction of movement out of the wood, as confined populations are often denser
than unrestricted populations due to inhibited dispersal (Krebs et al. 1973).
The estimates using minimum number alive methods were probably underestimates given
the assumption that all animals in an area were sampled. This was not the case as new
animals were always caught on the last day of trapping.
Recaptures of individuals in both species from previous trapping sessions were low
throughout the course of this study. This turn-over of the population may be attributed to
high mortality between sessions although this may not necessarily be the case, as wood mice
and bank voles tend to have high survival rates in autumn and winter (Tanton 1965, Smyth
1968, Flowerdew 1974). The animals that were recaptured from previous sessions were in
the sub-adult or adult age class. Immigration and emigration may account for this turn-over,
if this was the case then it indicates that possible dispersal movements were still occurring
throughout the autumn and winter (Watts 1969). A more likely scenario is the considerable
edge effects associated with small trapping grids (Crawley 1970), although without sufficient
data this is mere speculation.
A higher number of bank voles than wood mice perished in the traps, Tanton (1969)
associated mortality in traps with weight and found that adult mice and voles at a weight
below 12-13 g usually perished in traps during winter. This may have been the case for the
three mouse mortalities as both females which died were below 13g and the juvenile was 7g.
This mortality associated with weight may explain four of the bank vole deaths, however the
other deaths occurred in individuals above 18g in weight. The deaths occurred when night
temperatures dropped below -5o, although temperature alone may not be the cause of these
mortalities, as voles are active at low temperatures (Kikkawa 1964, Smyth1966) and
significantly more voles survived these temperatures. Voles are a predominantly diurnal

33. 33
species (Greenwood 1978) and as a result of this activity pattern and trapping techniques
used in this study they had a greater chance of longer confinement in traps.
The stress associated with long confinement is a likely factor in trap mortality (Crawley 1970).
This mortality may be reduced by checking traps twice a day, once in the morning and once
in the evening.
The appearance of pygmy shrews S. minutus in the traps is likely a result of their foraging
behaviour and not as a result of being attracted to bait. Death was due to starvation as shrews
have a high metabolic rate and requires 125% of its body weight daily to survive (Churchfield
1990)
Most studies on small rodent populations show a preponderance of males caught during live
trapping, for both the wood mouse and the bank vole (Miller 1958, Crawley 1969). Miller
(1958) found that the proportion of males rose to its highest in summer and early autumn
and then dropped to equality with females in late autumn and winter. Montgomery (1979)
found that wood mice have a sexual bias in trap response, he found that male mice are more
likely to enter a trap that was previously occupied by a male while females are equally likely
to enter a trap that was occupied by either sex, and bank voles have no such bias. This sexual
bias for traps may explain the preponderance of males in live trap results. The trap revealed
sex ratio of this study differed from these findings with a ratio favouring females. The sex
ratio on Grid A was 1:1 for wood mice while Grid B had 1.4:1 ratio in favour of females.
The sex ratio for voles differed between January and February, 1.3:1 ratio in favour of males
in January and a 2.6:1 ratio in favour of females in February. Alibhai & Gipps (1985) recorded
that the sex ratio in bank voles favoured males with the greatest difference found at high
densities. This was not the case in this study with a greater proportion of the population
being females as was found by Tanton (1969). Kikkawa (1964) also recorded similar results
with a greater number of females caught for both species captured. The greater number of
female captures in this study, may be a result of overwintering in communal burrows which
usually contain more females than males (Wolten 1983, Ylönen & Viitala 1991). Fasola &
Canova (2000) have shown that prolonged removal studies for both wood mice and bank
voles tend to reduce the ratio of females to males in a population.
Wood mice show seasonal fluctuations in their population structure, young born late in the
season usually overwinter as immature sub-adults, in spring overwintered mice mature
rapidly to breed and juveniles enter the population in April and May. When breeding ceases
few of the years breeding stock survive (Newson 1963). Gurnell (1978) found that by August
virtually all old individuals had disappeared and were replaced by juveniles and sub-adults,
the findings in this study are comparable though the higher number of adults persisted until
September and a small proportion of adults made up the population in October. During the
breeding season resident animals hinder the settlement of immigrants and reduce juvenile
and sub-adult survival (Flowerdew 1974). Breeding was still occurring in September and this
spatial intolerant behaviour of established mice may have reduced the number of young mice

34. 34
that were trappable. The breeding season was considered over when less than 50% of
females were fecund (Montgomery 1981). This occurred by the end of October, which
coincided with an influx of sub-adults into the trappable population along with a small
number of juveniles.
The removal of adult mice resulted in significant shift in the population structure on Grid B.
Removal of male mice can increase the rate of recruitment in population (Flowerdew 1974)
and this seemed to be the case in October on Grid B which had a high proportion of adults
prior to removal. These trends in population structure have been recorded in numerous
papers (Tanton 1969, Watts 1969, Gurnell 1978). The populations of bank voles in this study
consisted of mostly sub-adults and a number of parous adults which is the usual population
structure found at this time of year (Crawley 1970, Tanton 1969, Newson 1963).
Both wood mice and bank voles are usually seasonal breeders (Smyth 1966). With the
breeding season usually commencing between February and April and ending by October or
November (Tanton 1965, 1969). Winter breeding can occur in woodlands particularly after a
good seed crop (Smyth 1966). The author observed breeding in September with an individual
mouse having birthed a litter of seven in a trap which was slightly above the average litter
size (Flowerdew 1985). Five adult females were in breeding condition in October, with three
lactating and one perforate the other female died in the trap. No sub-adults were fecund at
this time, this would suggest that the main breeding season had ended by this time. A small
number of females were perforate in January and February which may indicate that some
breeding occurred through the winter but without data for November and December this was
inconclusive. The proportion of fecund males rose through January and February which is to
be expected with the onset of sexual maturity in overwintered mice (Tanton1969, Flowerdew
1985).
Only four individual bank voles were captured in the first half of the study and the two females
trapped in September were fecund, both males were also fecund, which suggest that
breeding was taking place. Over 50% of males were fecund in the second half of the study
which suggests that some winter breeding did occur, as in the absence of winter breeding
male’s testes regress (Corbet & Harris 1991). The appearance of juvenile voles in January on
Grid B was an indication that breeding was at least occurring in November. Two female bank
voles had swollen abdomens in February which could possibly have been pregnancies
(Newson 1963). Both Tanton (1969) and Smyth (1966) recorded fecund females in February.
Bank vole breeding season is highly variable and the start of the breeding season can be
brought forward by abundant food and low densities (Eccard & Ylönen2001). It would seem
that there was an adequate supply of food in the wood to bring forward the breeding season.
In a removal experiment with A. flavicollis and A. sylvaticus Montgomery (1981), he revealed
that the removal of one species can induce earlier breeding in a population in the absence of
its congener, and the removal of the congener may have the same effect of as removing
conspecifics. Winter breeding occurs at low densities even if there is not an apparent

35. 35
abundance of food (Fairley & Comerton 1972). The reduction in density of A. sylvaticus on
Grid B did not induce any females to become fecund at the end of October on this Grid, and
in fact there were more fecund females on Grid A which had a higher population density at
this time. In contrast to Montgomery’s (1981) study, Fasola & Canova (2002) could not find
any significant trend in the effect removal had on the breeding populations of M. glareolus
and A. sylvaticus either within or between species over a two year removal experiment. Due
to the timing of this study coinciding with the end of the breeding season the removal of mice
had a negligible effect on the breeding population.
Male wood mice on Grid B maintained a higher mean weight than those found on Grid A,
although there was no significant difference in size based on hind foot length. This would
suggest that some factor, such as food abundance may have improved the fitness of these
individuals, although this trend was not recorded for females. Smyth (1968) and Newson
(1963) recorded similar results in a removal experiments of neighbouring populations, they
attributed these differences to slight differences in habitat. The fall in mean weight of females
and the males on Grid A is typical of over winter loss of weight due to cold weather (Tanton
1969). In contrast to wood mice, bank voles increased in mean weight from January to
February. This could be a result of the onset of an early breeding season associated with the
growth of uteri, embryos and mammary glands in females (Montgomery 1981).
The coefficients for association suggest that there is a weak negative spatial relationship
between bank voles and wood mice to a varying degree. It was interesting to note that the
only significant negative association occurred on Grid A. The ground cover at this site was
heterogeneous. Bank voles prefer areas of thick ground cover and move out of areas with
reduced ground cover in winter (Kikkawa 1964), whereas wood mice can be distributed
independently of ground cover (Miller 1958). These associations likely arose due to different
micro-habitat preferences rather than outright spatial segregation. Bank voles in this grid
were never trapped at stations that didn’t have thick ground cover surrounding the station
while mice were trapped at every station at some point over the course of the study, although
this was an arbitrary observation a more systematic approach to measuring micro-habitat
would be needed to test this hypothesis (Fasola & Canova 2000, Montgomery 1981).
Montgomery (1979) showed that mice and voles are more likely to visit traps that have
previously been occupied by conspecifics, and with the high competition for traps at this time,
results may have been biased.
Fasola & Canova (2000) found asymmetrical competition between the bank vole and the
wood mouse. Removal of the wood mouse resulted in an increase in population density of
the bank vole. Although the population increased after the removal of wood mice in this
experiment, it occurred on both trapping areas and the removal of wood mice .

36. 36
4.1 Conclusion
There was no evidence of competitive release exhibited by the bank vole population after the
removal of wood mice from the population. The density of mice was probably not sufficiently
reduced during the removal period to have an effect on the bank vole population. To improve
this experiment multiple removals of wood mice from the experimental grid would be
necessary to maintain a low population density in relation to the bank vole population.
Wood mice were easily able to replace the removed individuals as the population was
undergoing the typical autumn increase.
Population densities of wood mice and bank voles were higher than other estimates for
similar habitats, suggesting that the study site was a very productive habitat. Grid B
supported higher densities of both species, which was probably a result of thicker vegetation
and more abundant food due to the abundance of hazel in this area. This may have been the
reason for the greater weight found in males on this site. Inhibited dispersal out of the
woodland was probably another reason for this high density.
The mild autumn weather, good mast crop, and low population density of bank voles in
autumn may have facilitated some winter breeding in this species, breeding had certainly
started by February. Some winter breeding also occurred in the wood mouse population.
Acknowledgements
I would like to thank Dr. Colin Lawton, my project supervisor, for his help throughout the year.
Thanks to Eimear Louth for the lifts out to the wood. I would like to especially thank Josefin
Eriksson who persevered the cold weather with me. Lastly I would like to thank my parents
Cathal and Sheila for all the support they have given me throughout the year.

37. 37
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