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THE EFFECTS OF CLEARCUT SIZE ON THE BIRD COMMUNITY IN THE SECOND COLLEGE GRANT

J
joshmooney

Abstract. This study examines the effects of forest opening (clearcut) size on the surrounding forest-bird community with the objective of offering management suggestions for foresters who employ the clearcut method. I hypothesized that large and small clearcuts would have different effects on the forest-bird assemblage associated with each. I used the point-count method to assess bird abundance in clearcuts, on the edges, and 100 m into the forest from the edges of large and small clearcuts. I found that Neotropical migrant birds and forest-interior birds were the most affected by large clearcuts showing significantly lower abundance in forest areas 100 m from large clearcut edges than in forest areas 100 m from small clearcuts. Edge-open birds were more abundant in large clearcut openings and edges than in small clearcut openings and edges. Blue jays (an avian nest predator) were more abundant on the edges of large clearcuts than on the edges of small clearcuts. A recent study found that forest-interior bird abundance levels off after 100 m distance from small (0.4 ha) forest openings. This result combined with my findings suggest that small openings in the Second College Grant represent less of a disturbance to Neotropical migrants and forest-interior birds. Additionally, given higher abundances of an avian nest predator in large clearcuts, reproductive success could be much lower in areas associated with large clearcuts. Some species such as the White-throated Sparrow (Zonotrichia albicollis), however preferred large clearcuts suggesting that there are some benefits to overall bird abundance by including large clearcuts in a managed landscape.

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THE EFFECTS OF CLEARCUT SIZE ON THE BIRD COMMUNITY IN
               THE SECOND COLLEGE GRANT

                                       JOSHUA R. MOONEY

       Abstract. This study examines the effects of forest opening (clearcut) size on the
       surrounding forest-bird community with the objective of offering management
       suggestions for foresters who employ the clearcut method. I hypothesized that large and
       small clearcuts would have different effects on the forest-bird assemblage associated with
       each. I used the point-count method to assess bird abundance in clearcuts, on the edges,
       and 100 m into the forest from the edges of large and small clearcuts. I found that
       Neotropical migrant birds and forest-interior birds were the most affected by large
       clearcuts showing significantly lower abundance in forest areas 100 m from large clearcut
       edges than in forest areas 100 m from small clearcuts. Edge-open birds were more
       abundant in large clearcut openings and edges than in small clearcut openings and edges.
       Blue jays (an avian nest predator) were more abundant on the edges of large clearcuts
       than on the edges of small clearcuts. A recent study found that forest-interior bird
       abundance levels off after 100 m distance from small (0.4 ha) forest openings. This
       result combined with my findings suggest that small openings in the Second College
       Grant represent less of a disturbance to Neotropical migrants and forest-interior birds.
       Additionally, given higher abundances of an avian nest predator in large clearcuts,
       reproductive success could be much lower in areas associated with large clearcuts. Some
       species such as the White-throated Sparrow (Zonotrichia albicollis), however preferred
       large clearcuts suggesting that there are some benefits to overall bird abundance by
       including large clearcuts in a managed landscape.




INTRODUCTION


Balancing timber management with bird conservation is a difficult endeavor. However,
given recent evidence of declines in many Neotropical migrant bird species (Robbins et
al. 1989a, Askins et al. 1990) it has become increasingly important to manage forested
lands to maintain avifauna diversity and abundance. Threats to migratory land birds in
their North American breeding ground include habitat destruction, degradation,
fragmentation, and loss of habitat diversity (Germaine et al. 1997). Forest fragmentation
increases the ratio of edge habitat to forest interior. Historically, edge habitat has been

seen as beneficial to a wide array of forest dwelling wildlife (e.g., Leopold 1933);
however, edge zones may contribute to reduced populations of bird species that require
large areas of forest interior habitat (Gates and Gysel 1978, Ambuel and Temple 1983,
Robinson 1992).
   It is important to study Neotropical migratory birds because these birds provide a
useful model for developing general programs of land management and improving
current management schemes (Martin 1995). Neotropical migratory birds are model
organisms because: (1) they are ubiquitous; (2) they include a wide diversity of
coexisting species with varying ecologies; (3) they are sensitive to environmental
perturbations because they are mobile, short-lived, and differ in their environmental
requirements; and (4) their behaviors, demographic characters (fecundity, survival),
physiology, species interactions, and habitat use can be readily studied for many species
(Martin 1995). In addition, knowledge of their habitat requirements is critical to
conserving forest land to allow for successful breeding.
   Given that a dominant proportion of birds that breed in the Northeast are Neotropical
migrants (MacArthur 1959), it is possible that factors affecting survival in their wintering
habitat may be more important than events in the breeding habitat. Some research has
found that migrants are limited chiefly by events and conditions in tropical wintering
grounds (Lack 1968, Fretwell 1972, 1986, Morse 1980b, Alerstam and Hogstedt 1982,
Baillie and Peach 1992, Morton 1992, Rappole et al. 1992). On the other hand,
fragmentation of forest habitats in eastern North America has been strongly implicated as
one cause of reduced breeding success, and consequently lowered breeding densities of
some songbird populations (Robbins 1979, Whitcomb et al. 1981, Ambuel and Temple
1983, Wilcove and Whitcomb 1983, Lynch and Whigham 1984, Wilcove and Robinson
1990). Long-distance migrants seem to be limited by both breeding and wintering
habitat; however, there is no clear study suggesting one is more important than the other.
Sherry and Holmes (1995) develop the idea of summer and winter limitation for migrant
birds, stating that “migrant bird populations appear to be limited contemporaneously by
their need for quality habitats in which to maintain high fecundity in summer and in
which to maintain high survival in winter.” Changes in breeding habitat are therefore at
least one of the important factors that could have profound effects on overall populations
of Neotropical migrant birds. Thus, it is important to study how forest management in
northeastern North America affects bird communities.
   Factors associated with timber harvesting that affect bird communities can be
generally categorized in one of two ways: factors associated with increased edge resulting
from habitat fragmentation (“edge effects”), and those associated with reduced area of
suitable forest habitats (“area effects”) (Welsh and Healy 1993). Many studies have been
conducted in highly fragmented forests and have found that fragment area is a major
factor in determining the number and diversity of bird species present (Blake and Karr
1987, Askins et al. 1990). In addition, Donovan et al. (1995) found that small forest
fragments may be sink habitats for some species of Neotropical migrants requiring
immigration from nearby source areas to maintain populations.
   Many areas of the Northeast include large tracts of contiguous forest. This is quite
different from many of the highly fragmented regions in which the aforementioned
studies were conducted. In areas of continuous forest, some studies have found increased
bird species richness and density near forest edge created by clearcuts, rivers, fields, or
power lines (McElveen 1979, Strelke and Dickson 1980, Hansson 1983, Germaine et al.
1997). However, other studies have not found such relationships (Kroodsma 1982, Small
and Hunter 1989).
   Bird species diversity is often higher in managed forest areas (where timber
harvesting occurs – clearcutting, selective harvesting...) than in reserved forest areas due
to the increase in habitat diversity (Welsh and Healy 1993). However, it is important to
take this question to the next level and ask how bird species diversity (and abundance) is
affected by clearcut size in a managed area. Simply because some studies have shown
that bird species diversity is higher in areas where timber management occurs (Germaine
et al. 1997), should we assume that creating forest openings will increase diversity? The
answer to this question will have implications for timber management in the Northeast
and throughout the country. In some cases managing for diversity is not always the
answer. When a sensitive or endangered species is present, it may be more prudent to
ensure its survival. Clearcutting is a relatively common practice in the Second College
Grant and elsewhere in the Northeast. The size of clearcuts varies to a great degree.
Thus, it is important to look at the effects of clearcut size on bird communities, and to
come to some conclusion as to which size affords the greatest benefits.
   Communication between researchers and forest managers is essential to designing
appropriate wildlife management schemes. The Second College Grant, where this study
was conducted, is a perfect forum for this type of discourse. Currently, the forester is
implementing a Wildlife Project, the goal of which is to manage the Grant for wildlife
while still harvesting timber. In order to implement a sound program the forester needs
input from researchers. This study will assist the Wildlife Project by providing
suggestions for bird management.
   This study reports results of point count censuses of birds conducted in the Second
College Grant, a 10,930 ha parcel of land in northern New Hampshire owned by
Dartmouth College. The objectives were: (1) to examine the differences in bird diversity,
abundance, and species assemblage between large and small clearcuts, (2) to look at these
differences in detail by examining the effects of clearcut size on bird guilds across a
habitat gradient from inside the clearcuts to the edge to 100 m into the surrounding forest,
and (3) to focus specifically on how Neotropical migratory birds were affected. I tested
for these effects by comparing species diversity, similarity of species composition, and
the abundance of birds in several guilds in large (6.0 – 7.0 ha) and small (0.3 – 1.0 ha)
clearcuts. These variables were compared within the clearcuts, on the edges of the
clearcuts, and 100 m into the forest from the clearcut.


METHODS
STUDY AREAS


This study was conducted from 20 June - 30 July 1997 in the Second College Grant in
northern Coos County, NH. I selected four small and four large structurally similar study
sites by field reconnaissance. These sites were dominated by hardwood trees, but also
included coniferous tree species. Hardwood species included sugar maple (Acer
saccharum), white and yellow birch (Betula papyrifera, B. alleghaniensis), and beech
(Fagus grandifolia). Coniferous species were red spruce (Picea rubens), balsam fir
(Abies balsamea), and some Eastern hemlock (Tsuga canadensis). Aerial photos were
examined to ensure that substantial areas of forest surrounded each cut. The clearcuts
studied were of similar age; all had been harvested within 5 years. Small sites ranged in
size from approximately 0.3 - 1.0 ha. Large sites ranged in size from 6.0 - 7.0 ha (sizes
determined using a Global Positioning System and Trimble’s Pfinder GIS software).


VEGETATION MEASUREMENTS


To test whether my sites were structurally similar, I measured basal area of all trees larger
than 7.5 cm in an 11 m radius around my point count sites which were inside the
clearcuts, on the edges of the clearcuts, and 100 m into the forest from the clearcut edges
(see Figure 1). These measurements were taken in the four large sites and in the four
small sites. One of the large sites was excluded from the study because it differed
strongly in forest composition being composed chiefly of spruce.




BIRD CENSUSING
I recorded all birds heard or seen within a 50 m radius of my point count site during a 6
minute period. Point counts were conducted between 05:00 and 08:00 from 29 June
through 19 July. Three census points were established at each site (Fig. 1) and marked by
flagging. One point was located inside the clearcut, 100 m from the edge. The second
was located on the edge. The third was located 100 m into the interior forest. The points
were situated in a straight line to avoid overlap. Each census point was visited four times
throughout the course of the study period.




               Figure 1. Clearcut, Edge, and Forest point count layout for a typical

               clearcut site in the Second College Grant, New Hampshire.



BIRD GUILDS
   To look at finer-scale differences in how birds were affected by clearcut size, I first
categorized the common bird species observed in the Grant on the basis of migratory
group. There were three categories: Neotropical migrants, Nearctic migrants, and
resident species. I then separated these species into groups based on habitat preference.
Forest-interior birds are those species with breeding territories concentrated away from
the forest edge. Interior-edge birds are species with territories both near the forest edge

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THE EFFECTS OF CLEARCUT SIZE ON THE BIRD COMMUNITY IN THE SECOND COLLEGE GRANT

  • 1. THE EFFECTS OF CLEARCUT SIZE ON THE BIRD COMMUNITY IN THE SECOND COLLEGE GRANT JOSHUA R. MOONEY Abstract. This study examines the effects of forest opening (clearcut) size on the surrounding forest-bird community with the objective of offering management suggestions for foresters who employ the clearcut method. I hypothesized that large and small clearcuts would have different effects on the forest-bird assemblage associated with each. I used the point-count method to assess bird abundance in clearcuts, on the edges, and 100 m into the forest from the edges of large and small clearcuts. I found that Neotropical migrant birds and forest-interior birds were the most affected by large clearcuts showing significantly lower abundance in forest areas 100 m from large clearcut edges than in forest areas 100 m from small clearcuts. Edge-open birds were more abundant in large clearcut openings and edges than in small clearcut openings and edges. Blue jays (an avian nest predator) were more abundant on the edges of large clearcuts than on the edges of small clearcuts. A recent study found that forest-interior bird abundance levels off after 100 m distance from small (0.4 ha) forest openings. This result combined with my findings suggest that small openings in the Second College Grant represent less of a disturbance to Neotropical migrants and forest-interior birds. Additionally, given higher abundances of an avian nest predator in large clearcuts, reproductive success could be much lower in areas associated with large clearcuts. Some species such as the White-throated Sparrow (Zonotrichia albicollis), however preferred large clearcuts suggesting that there are some benefits to overall bird abundance by including large clearcuts in a managed landscape. INTRODUCTION Balancing timber management with bird conservation is a difficult endeavor. However, given recent evidence of declines in many Neotropical migrant bird species (Robbins et al. 1989a, Askins et al. 1990) it has become increasingly important to manage forested lands to maintain avifauna diversity and abundance. Threats to migratory land birds in their North American breeding ground include habitat destruction, degradation, fragmentation, and loss of habitat diversity (Germaine et al. 1997). Forest fragmentation increases the ratio of edge habitat to forest interior. Historically, edge habitat has been seen as beneficial to a wide array of forest dwelling wildlife (e.g., Leopold 1933); however, edge zones may contribute to reduced populations of bird species that require
  • 2. large areas of forest interior habitat (Gates and Gysel 1978, Ambuel and Temple 1983, Robinson 1992). It is important to study Neotropical migratory birds because these birds provide a useful model for developing general programs of land management and improving current management schemes (Martin 1995). Neotropical migratory birds are model organisms because: (1) they are ubiquitous; (2) they include a wide diversity of coexisting species with varying ecologies; (3) they are sensitive to environmental perturbations because they are mobile, short-lived, and differ in their environmental requirements; and (4) their behaviors, demographic characters (fecundity, survival), physiology, species interactions, and habitat use can be readily studied for many species (Martin 1995). In addition, knowledge of their habitat requirements is critical to conserving forest land to allow for successful breeding. Given that a dominant proportion of birds that breed in the Northeast are Neotropical migrants (MacArthur 1959), it is possible that factors affecting survival in their wintering habitat may be more important than events in the breeding habitat. Some research has found that migrants are limited chiefly by events and conditions in tropical wintering grounds (Lack 1968, Fretwell 1972, 1986, Morse 1980b, Alerstam and Hogstedt 1982, Baillie and Peach 1992, Morton 1992, Rappole et al. 1992). On the other hand, fragmentation of forest habitats in eastern North America has been strongly implicated as one cause of reduced breeding success, and consequently lowered breeding densities of some songbird populations (Robbins 1979, Whitcomb et al. 1981, Ambuel and Temple 1983, Wilcove and Whitcomb 1983, Lynch and Whigham 1984, Wilcove and Robinson 1990). Long-distance migrants seem to be limited by both breeding and wintering habitat; however, there is no clear study suggesting one is more important than the other. Sherry and Holmes (1995) develop the idea of summer and winter limitation for migrant birds, stating that “migrant bird populations appear to be limited contemporaneously by their need for quality habitats in which to maintain high fecundity in summer and in
  • 3. which to maintain high survival in winter.” Changes in breeding habitat are therefore at least one of the important factors that could have profound effects on overall populations of Neotropical migrant birds. Thus, it is important to study how forest management in northeastern North America affects bird communities. Factors associated with timber harvesting that affect bird communities can be generally categorized in one of two ways: factors associated with increased edge resulting from habitat fragmentation (“edge effects”), and those associated with reduced area of suitable forest habitats (“area effects”) (Welsh and Healy 1993). Many studies have been conducted in highly fragmented forests and have found that fragment area is a major factor in determining the number and diversity of bird species present (Blake and Karr 1987, Askins et al. 1990). In addition, Donovan et al. (1995) found that small forest fragments may be sink habitats for some species of Neotropical migrants requiring immigration from nearby source areas to maintain populations. Many areas of the Northeast include large tracts of contiguous forest. This is quite different from many of the highly fragmented regions in which the aforementioned studies were conducted. In areas of continuous forest, some studies have found increased bird species richness and density near forest edge created by clearcuts, rivers, fields, or power lines (McElveen 1979, Strelke and Dickson 1980, Hansson 1983, Germaine et al. 1997). However, other studies have not found such relationships (Kroodsma 1982, Small and Hunter 1989). Bird species diversity is often higher in managed forest areas (where timber harvesting occurs – clearcutting, selective harvesting...) than in reserved forest areas due to the increase in habitat diversity (Welsh and Healy 1993). However, it is important to take this question to the next level and ask how bird species diversity (and abundance) is affected by clearcut size in a managed area. Simply because some studies have shown that bird species diversity is higher in areas where timber management occurs (Germaine et al. 1997), should we assume that creating forest openings will increase diversity? The
  • 4. answer to this question will have implications for timber management in the Northeast and throughout the country. In some cases managing for diversity is not always the answer. When a sensitive or endangered species is present, it may be more prudent to ensure its survival. Clearcutting is a relatively common practice in the Second College Grant and elsewhere in the Northeast. The size of clearcuts varies to a great degree. Thus, it is important to look at the effects of clearcut size on bird communities, and to come to some conclusion as to which size affords the greatest benefits. Communication between researchers and forest managers is essential to designing appropriate wildlife management schemes. The Second College Grant, where this study was conducted, is a perfect forum for this type of discourse. Currently, the forester is implementing a Wildlife Project, the goal of which is to manage the Grant for wildlife while still harvesting timber. In order to implement a sound program the forester needs input from researchers. This study will assist the Wildlife Project by providing suggestions for bird management. This study reports results of point count censuses of birds conducted in the Second College Grant, a 10,930 ha parcel of land in northern New Hampshire owned by Dartmouth College. The objectives were: (1) to examine the differences in bird diversity, abundance, and species assemblage between large and small clearcuts, (2) to look at these differences in detail by examining the effects of clearcut size on bird guilds across a habitat gradient from inside the clearcuts to the edge to 100 m into the surrounding forest, and (3) to focus specifically on how Neotropical migratory birds were affected. I tested for these effects by comparing species diversity, similarity of species composition, and the abundance of birds in several guilds in large (6.0 – 7.0 ha) and small (0.3 – 1.0 ha) clearcuts. These variables were compared within the clearcuts, on the edges of the clearcuts, and 100 m into the forest from the clearcut. METHODS
  • 5. STUDY AREAS This study was conducted from 20 June - 30 July 1997 in the Second College Grant in northern Coos County, NH. I selected four small and four large structurally similar study sites by field reconnaissance. These sites were dominated by hardwood trees, but also included coniferous tree species. Hardwood species included sugar maple (Acer saccharum), white and yellow birch (Betula papyrifera, B. alleghaniensis), and beech (Fagus grandifolia). Coniferous species were red spruce (Picea rubens), balsam fir (Abies balsamea), and some Eastern hemlock (Tsuga canadensis). Aerial photos were examined to ensure that substantial areas of forest surrounded each cut. The clearcuts studied were of similar age; all had been harvested within 5 years. Small sites ranged in size from approximately 0.3 - 1.0 ha. Large sites ranged in size from 6.0 - 7.0 ha (sizes determined using a Global Positioning System and Trimble’s Pfinder GIS software). VEGETATION MEASUREMENTS To test whether my sites were structurally similar, I measured basal area of all trees larger than 7.5 cm in an 11 m radius around my point count sites which were inside the clearcuts, on the edges of the clearcuts, and 100 m into the forest from the clearcut edges (see Figure 1). These measurements were taken in the four large sites and in the four small sites. One of the large sites was excluded from the study because it differed strongly in forest composition being composed chiefly of spruce. BIRD CENSUSING
  • 6. I recorded all birds heard or seen within a 50 m radius of my point count site during a 6 minute period. Point counts were conducted between 05:00 and 08:00 from 29 June through 19 July. Three census points were established at each site (Fig. 1) and marked by flagging. One point was located inside the clearcut, 100 m from the edge. The second was located on the edge. The third was located 100 m into the interior forest. The points were situated in a straight line to avoid overlap. Each census point was visited four times throughout the course of the study period. Figure 1. Clearcut, Edge, and Forest point count layout for a typical clearcut site in the Second College Grant, New Hampshire. BIRD GUILDS To look at finer-scale differences in how birds were affected by clearcut size, I first categorized the common bird species observed in the Grant on the basis of migratory group. There were three categories: Neotropical migrants, Nearctic migrants, and resident species. I then separated these species into groups based on habitat preference. Forest-interior birds are those species with breeding territories concentrated away from the forest edge. Interior-edge birds are species with territories both near the forest edge
  • 7. and in the interior. Edge-open species are those with territories both in edge areas and in openings (Whitcomb et al. 1981). Thirdly, I classified the species into foraging guilds based on information in the literature (Ehrlich et al. 1988). Foliage-gleaners are those species that forage insects from the leaves of trees. Ground-foragers are those species that manipulate the forest floor in search of insects, and bark-probers probe into the bark of trees with their bills for insects. DATA ANALYSIS Basal area, and diversity (Simpson’s Index) were analyzed with an ANOVA model that included clearcut size, location (clearcut, edge, or forest), clearcut nested within clearcut size, and clearcut size x location. Abundance within guilds (migratory group, habitat association, and foraging strategy) was analyzed with an ANOVA model that included all the above terms and additionally: guild, size x guild, and location x guild. All terms were treated as fixed effects. Similarity between large and small habitat groups was determined using the Bray- Curtis (1957) measurement of species composition and relative abundance measures as comparison criteria (Germaine et al. 1997), where: " similarity = $ (∑ 2Wi ) % ( A + B )' × 100% # & and: Wi = lower abundance of each species i present in both plots, A = number of individuals present in assemblage 1 and B = number of individuals present in assemblage 2.
  • 8. RESULTS Basal area of coniferous trees was similar in large and small sites (Table 1; Fig 2). Basal area of hardwood trees was also similar in large and small sites (Table 1; Fig 3). Table 1. Summary of mean squares and F statistics for ANOVA models run with basal area of coniferous trees and hardwood trees, and bird diversity as variables. Basal Area (H) Basal Area (C) Bird Diversity df MS F MS F MS F Size 1 4.52 1.89 1.66 0.73 1.24 0.18 Site (Site) 5 1.39 0.58 1.06 0.46 3.88 0.58 Location 2 6.94 2.90 2.48 1.08 53.01 7.92** Size x Location 2 0.77 0.32 0.07 1.03 0.91 0.14 Error 10 2.40 -- 2.28 -- 6.69 -- P < 0.05; ** P < 0.01; *** P < 0.001 (H) = Hardwood Trees; (C) = Coniferous Trees .)tseroF dna ,egdE ,tucraelC( .)tseroF dna ,egdE ,tucraelC( etis hcae ta snoitacol eerht ta nekat serusaem etis hcae ta snoitacol eerht ta nekat serusaem aera lasaB .)4=n( setis llams dna )3=n( setis egral aera lasaB .)4=n( setis llams dna )3=n( setis egral ni seert suorefinoc fo aera lasab naeM .2 erugiF ni seert doowdrah fo aera lasab naeM .3 erugiF tucraelC egdE tseroF tucraelC egdE tseroF 0 0 ).mc .qs( aera lasab naeM ).mc .qs( aera lasab naeM 5.0 5.0 1 1 5.1 5.1 llamS llamS egraL egraL 2 2 I registered a total of 39 bird species (Appendix I) within 50 m of 21 point count stations. Only species for which I recorded more than five individuals throughout the data collection period (18 species) were included in the analyses. In addition, point counts were inappropriate for censusing some species (Ruffed Grouse, Cedar Waxwing,
  • 9. Chimney Swifts, hawks), consequently these birds were not included (see Appendix I for list of all birds encountered). Bird species diversity did not differ in large and small sites in the three locations (clearcut, edge, forest) (Table 1; Fig 4). However, there was a highly significant effect of location when diversity values of large and small sites were pooled (Table 1; Fig 4). Diversity inside the clearcut (in both large and small sites) was significantly lower than edge and forest values (F1,10 = 9.52, P = 0.01). There was no difference between diversity in edge and forest areas (F1,10 = 0.41, P = 0.54). tucraelC egdE tseroF .)ES ± naem( 7991 ,erihspmaH weN ,tnarG 0 egelloC dnoceS eht ni setis )4 = n( llams dna )3 = n( 1 egral rof seirogetac tatibah eerht ni )xednI ytisreviD s'nospmiS( seiceps drib fo ytisrevid naeM .4 erug2 iF 3 4 )xednI s'nospmiS( ytisreviD naeM 5 6 7 8 9 01 llamS 11 21 egraL Though diversity did not differ with clearcut size, the percent similarity between large and small sites was low (Table 2). The differences in bird species present can be seen by looking at the species common in each type of habitat (Table 3). Inside large and small
  • 10. clearcuts, few of the species overlap suggesting very different bird assemblages. In edge areas of large and small clearcuts more species overlap, and in forest areas a majority of the species overlap indicating the relative similarity between the assemblages. Table 2. Percent similarity within the bird community in clearcuts, edge zones, and interior forest zones of large and small clearcut plots, Second College Grant, New Hampshire, 1997. Small Clearcut Small Edge Small Forest Large Clearcut 0.195 -- -- Large Edge -- 0.275 -- Large Forest -- -- 0.325 Table 3. Representation of where each species was relatively common (birds/site > 0.10) in the Second College Grant, New Hampshire. Point counts were conducted inside clearcuts, on the edges of clearcuts and in the forest 100 m from clearcut edges. Bird species are separated by migratory group. Small site: Large Site: Small Site: Large site: Small Site: Large Site: Migrant Group/Species Clearcut Clearcut Edge Edge Forest Forest * Neotropical Migrants Blackburnian warbler X X X Black-throated blue warbler X X X X Black-throated green warbler X X X Common yellowthroat X X X Hermit thrush X X X X Magnolia warbler X X X Mourning warbler X X X Northern parula X X X X Ovenbird X X X Red-eyed vireo X X X X X Nearctic Migrants White-throated sparrow X X X X Winter wren X X X X Yellow-bellied sapsucker X Yellow-rumped warbler X Residents Black-capped chickadee X Blue jay X X Dark-eyed junco X X X X X X Golden-crowned kinglet X X X X The finer details of these differences in similarity are revealed by separating the species observed into several groupings and analyzing the differences. The most interesting result from analysis of the effects of size and location on the migratory groupings (Neotropical migrant, Nearctic migrant, or Resident) was that there were
  • 11. significantly more Neotropical migrant birds in the interior forest surrounding small clearcuts than in the interior forest surrounding large clearcuts (F1,40 = 10.64, P = 0.002; Figure 5). The other migratory status groups did not show any significant relationships with size and location. .50.0 < P * .)ES ± naem( 7991 ,erihspmaH weN ,tnarG egelloC dnoceS eht ni setis )4 = n( llams dna )3 = n( egral rof seirogetac tatibah eerht ni sdrib tnargim fo ecnadnuba naeM .5 erugiF tucraelC egdE tseroF 0 )etis/sdrib#( ecnadnuba naeM 1.0 2.0 3.0 4.0 llamS 5.0 * egraL The second grouping was based on habitat associations similar to those of Whitcomb et al. (1981) and Germaine et al. (1997). There were significantly more forest-interior birds in the forest 100 m from small clearcuts than in the forest 100 m from large clearcuts (F1,40 = 5.64, P = 0.02; Figure 6). In contrast, there were significantly more edge-open birds associated with large clearcuts (F1,40 = 5.05, P = 0.03; Figure 7) and clearcut edges (F1,40 = 8.07, P = 0.007; Figure 7) than with small clearcuts and clearcut edges. Interior-edge birds showed a preference for interior and edge zones as expected, while exhibiting low abundance in clearcut areas (Appendix II). There was no effect of clearcut size for interior-edge species (Table 4).
  • 12. .50.0 < P * .)ES ± naem( 7991 ,erihspmaH .50.0 < P * .)ES ± naem( 7991 ,erihspmaH weN ,tnarG egelloC dnoceS eht ni setis )4 = n( weN ,tnarG egelloC dnoceS eht ni setis )4 = llams dna )3 = n( egral rof seirogetac tatibah eerht n( llams dna )3 = n( egral rof seirogetac tatibah eerht ni sdrib roiretni-tserof fo ecnadnuba naeM .6 erugiF ni sdrib nepo-egde fo ecnadnuba naeM .7 erugiF tucraelC egdE tseroF tucraelC egdE tseroF 0 0 )etis/sdrib#( ecnadnuba naeM 1.0 )etis/sdrib#( ecnadnuba naeM 1.0 2.0 2.0 3.0 3.0 4.0 llamS 4.0 llamS 5.0 * egraL egraL 6.0 5.0 * * Table 4. Summary of mean squares and F statistics for ANOVA models run with migratory group, habitat association, and foraging strategy as variables. Migratory Group Habitat Association Foraging Strategy df MS F MS F MS F Size 1 0.89 0.16 0.48 1.20 0.89 0.14 Site (Site) 5 2.17 0.40 0.43 1.08 2.17 0.34 Location 2 121.52 22.34*** 10.42 26.30*** 121.52 19.27*** Guild 2 234.51 43.11*** 0.57 1.44 359.85 57.07*** Size x Location 2 31.20 5.73** 1.69 4.26* 31.20 4.95* Size x Guild 2 10.96 2.01 2.13 5.37** 7.35 1.16 Location x Guild 4 76.62 14.08*** 4.70 11.85*** 55.57 8.81*** Error 44 5.44 -- 0.40 - 6.31 - *P < 0.05; ** P < 0.01; *** P < 0.001 The third grouping was based on foraging strategies (Ehrlich et al. 1988). The abundance of foliage-gleaning birds increased from clearcut to forest (F4,40 = 8.80 P < 0.001; Figure 8). Ground-foraging birds were most abundant in clearcut areas and edge areas of large sites and in interior areas of small sites (Figure 9). There were no Yellow- bellied Sapsuckers (the only commonly encountered bark-probing species) associated with large clearcuts and they were most abundant in interior areas associated with small clearcuts (see Appendix II).
  • 13. large clearcuts than in small sites (Figure 10). predators. They were more abundant in edge areas and interior areas associated with Finally, I looked at the abundance of blue jays, which are potentially important nest egraL 5.0 5.0 llamS * egraL * 4.0 llamS 4.0 3.0 3.0 * 2.0 2.0 1.0 1.0 )etis/sdrib#( ecnadnuba naeM )etis/sdrib#( ecnadnuba naeM 0 0 tseroF egdE tucraelC tseroF egdE tucraelC sdrib gnigarof-dnuorg fo ecnadnuba naeM .9 erugiF sdrib gninaelg-egailof fo ecnadnuba naeM .8 erugiF llams dna )3 = n( egral rof seirogetac tatibah eerht ni llams dna )3 = n( egral rof seirogetac tatibah eerht ni weN ,tnarG egelloC dnoceS eht ni setis )4 = n( weN ,tnarG egelloC dnoceS eht ni setis )4 = n( erew snaem - * .)ES ± naem( 7991 ,erihspmaH .)ES ± naem( 7991 ,erihspmaH .)50.0 < P( tnereffid yllacitsitats
  • 14. .50.0 < P * .)ES ± naem( 7991 ,erihspmaH weN ,tnarG egelloC dnoceS eht ni setis )4 = n( llams dna )3 = n( egral rof seirogetac tatibah eerht ni )srotaderp tsen( syaj eulb fo ecnadnuba naeM .01 erugiF tucraelC egdE tseroF 0 )etis/sdrib#( ecnadnuba naeM 1.0 2.0 3.0 4.0 5.0 6.0 * llamS 7.0 egraL DISCUSSION Clearcuts of different sizes affect the forest-bird community in different ways. Neotropical migrant bird species and forest-interior species appear to be the most sensitive bird groups to large (6.0 - 7.0 ha) clearcut disturbances in the Grant. Nearly all bird species in eastern North America that showed declining trends from the late 1940s until the late 1980s were Neotropical migrants or forest-interior birds and interior-edge birds (Whitcomb et al. 1981). In light of this finding and others showing declining trends in migratory songbirds and birds associated with forest-interior (Askins et al. 1990, Robbins et al. 1989a) management strategies in the Second College Grant should reflect a concern for these groups of birds. Forest-interior and bark-probing birds are able to maintain relatively high abundances around small clearcuts while they are at low abundance or are absent from areas associated with large clearcuts. In addition, though ground-foraging birds showed high abundances in large clearcuts and edge associated with large clearcuts, they also
  • 15. maintained high abundances in forest 100 m from small clearcuts. This suggests that large open areas are not necessary for maintaining high abundances of these species. Edge-open birds did prefer large clearcuts and edge areas of large clearcuts over small clearcuts and edge areas. These species were also present in small clearcuts (Table 3) suggesting that overall bird diversity in the Second College Grant does not benefit from the presence of large openings. The overall abundance of birds, however, is enhanced by the presence of large clearcut openings given that species such as the White- throated Sparrow (Zonotrichia albicollis), Red-eyed Vireo (Vireo olivaceus), and Winter Wren (Troglodytes troglodytes) were much more abundant in large clearcuts than in small clearcuts (Appendix II). Of course, just how large openings should be to support a higher abundance of edge-open birds must be determined for the Grant. This study suggests that 6.0 – 7.0 ha clearcuts are more beneficial to edge-open species although clearcuts smaller than 6.0 ha may support equally high abundances of edge-open species. This question is important given that there may be some intermediate size clearcut that Neotropical migrants, and forest-interior birds and edge-open species can all utilize. Large clearcuts represent a major disturbance to the local landscape in which they are carried out. Relative to small clearcuts, they destroy a larger amount of habitat for forest- interior birds. Most importantly, it appears from my data that the effects of these large disturbances extend further into the surrounding forest than the effects of a small clearcut. Germaine et al. (1997) found that forest-interior bird abundance leveled off after 100 m distance from small (0.4 ha) forest openings. My abundance measures 100 m from small clearcuts in the Grant may therefore be representative of areas even further from clearcuts (interior forest). If this is so, birds must be further away from large clearcuts than from small clearcuts in order to maintain high abundance. Of course, we must note the cumulative effect of many small clearcuts totaling to equal the size of one large clearcut. It could be possible that many small clearcuts disturb more habitat for Neotropical migrants and forest-interior birds. To assess this we must know the distance from a large
  • 16. clearcut at which the abundance of migrants and forest-interior birds equals the abundance 100 m away from a small clearcut. This information would be very useful for developing a strategy to manage for Neotropical migrants and forest-interior birds which are showing declines recently. Blue jays, potentially important nest predators, are also more of a concern in large clearcuts than in small clearcuts in the Grant. Their greater abundance in large sites may explain why forest-interior birds and Neotropical migrants were more abundant in forest 100 m from small clearcuts than in forest 100 m from large clearcuts. In addition, forest associated with large clearcuts may be more detectable or exploitable than forest adjacent to small clearcuts by nest predators such as chipmunks, squirrels and avian nest predators than the more discontinuous edges of small clearcuts (Germaine et al. 1997). An increased presence of nest predators could have serious implications for reproductive success. I did not measure nest success, but the differences I saw in abundance may be even more pronounced in terms of reproductive success. Measuring reproductive success in and around large and small clearcuts would provide land managers with an important insight into the nature of the effects of clearcut size on the bird community. This study shows the importance of looking beyond diversity measures as an indication of management success. If management in the Grant based assessment of different strategies strictly on the diversity of the bird assemblage present, much would be overlooked. For instance, sensitive groups such as Neotropical migrants and forest- interior species showing population declines would not show up in such an analysis. To manage for Neotropical migrant and forest-interior species in the Second College Grant, managers should limit clearcut size. Additional timber could be harvested using selective cutting although the effects of this practice should be examined in future studies as well. Such a strategy may be prudent given the nature of surrounding timber management schemes. The land immediately east of the Grant is owned by a large paper mill that harvests large tracts of forest (much larger than 6.0 – 7.0 ha) using the clearcut
  • 17. method. Thus, in terms of larger landscape dynamics, edge-open birds may have an abundance of habitat outside the Grant’s borders while Neotropical migrants and forest- interior birds are more limited. To manage for overall bird abundance within the Grant, management strategies should include large and small clearcuts. This would benefit the most number of species in terms of abundance. Again it is important to remember, however, that reproductive success may be lower in forest associated with large clearcuts and one must be hesitant in adopting a strategy that includes large clearcuts if the goal is to manage for edge-open bird abundance. Of course, it is important to consider the habitat requirements of wildlife other than birds such as moose, bear, deer, and aquatic organisms. In addition, timber production and the economics driving it are important factors to consider. Future studies should attempt to synthesize concerns for other organisms with my results for forest birds to provide the forester at the Grant with the necessary information to implement a sound and effective Wildlife Program. In addition future work could attempt to determine whether many small clearcuts which add up to the same amount of area as one large clearcut would be more beneficial to the bird assemblage. LITERATURE CITED Alerstam, T., and G. Hogstedt. 1982. Bird migration and reproduction in relation to habitats for survival and breeding. Ornis. Scand. 13:25-37. Ambuel, B., and S. A. Temple. 1983. Area-dependent changes in bird communities and vegetation of southern Wisconsin forests. Ecology 64:1057-1068. Askins, R. A., J. F. Lynch, and R. Greenberg. 1990. Population declines in migratory birds in eastern North America. Current Ornithol. 7:1-57. Baillie, S.R., and W. J. Peach. 1992. Population limitation in Palearctic-African migrant passerines. Ibis 134 (Suppl. 1): 120-132. Blake, J. G., and J. R. Karr. 1987. Breeding birds of isolated woodlots: area and habitat relationships. Ecology 68:1724-1734. Bray, J. R., and J. T. Curtis. 1957. An ordination of the upland forest communities of southern Wisconsin. Ecol. Monogr. 27:325-349.
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