MadanRiva_Thesis

Riva T. Madan Wild Turkey Population Change Spring 2015
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Predictors of Wild Turkey (Meleagris gallopavo) Population Change
in California from 1972 to 2013
Riva T. Madan
ABSTRACT
Wild turkeys never lived in California until humans began introducing them around 1908. The
success of this introduced species was variable at first, but in the 1990s there was a noticeable
increase in their population and range. The reasons behind what drove these population changes
are unknown, especially with only a few studies on wild turkeys in California. With Breeding Bird
Survey, National Land Cover Database, PRISM Climate Group, and California Department of Fish
and Wildlife data, I used a zero-inflated poisson mixed model to determine the influence of
climate, translocations, hunting, and land cover on wild turkey population change. I ran a separate
model for three land cover distances. I calculated population change statewide and at individual
routes using a generalized linear model. Wild turkey populations increased 10% per year statewide
from 1972 to 2013. Hunting was the most influential predictor of population change showing that
populations are increasing even as hunting increases. Land cover was a significant predictor, but
the effects of different land cover types varied with the scale looked at. Urban land cover was
positively related to population change only at 1 and 10 km distances. At the 5 km distance, forest,
grassland, and agriculture were inversely related to population change. This study suggests
increasing urban land cover, which may increase food supply, can increase wild turkey
populations. However, the results also suggest that other factors that are not included in this model
have large influences on wild turkey population change.
KEYWORDS
introduced species, Breeding Bird Survey, zero inflated poisson mixed model, land cover
change, ArcGIS

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INTRODUCTION
Humans often accidentally or intentionally transport species to new locations beyond their
natural range. If the species successfully establishes itself, the species becomes an introduced
species. At this point their populations may grow very large and spread over a large area. If the
introduced species has both expanded in range and is causing ecological impacts, the species is
considered invasive (Duncan et al. 2003). Large populations of introduced species can also be
destructive to human activities, making the species become a pest (Baldwin et al. 2012). Wild
turkeys (Meleagris gallopavo) are an example of an introduced species in California that has
expanded in range and increased in population. Although they are potentially a pest, wild turkeys
are not considered invasive due to lack of research on its ecological impacts.
The wild turkey never originally lived in California. Is native range is east of the
Mississippi River, in the southwest United States (Arizona, New Mexico, and Texas), and Mexico
(Gardner et al. 2004). In the late 1800s, wild turkey populations in their native range declined as a
result of deforestation and hunting (Dickson 1992; Barret and Kucera 2005). As a reaction to the
fear of turkeys going extinct, turkeys were introduced into every state except Alaska (Dickson
1992). The first record of wild turkey introduction in California was in 1877, when private ranchers
introduced the wild turkey on Santa Cruz Island. The next recorded introduction was in 1908 when
the California Department of Fish and Wildlife (CDFW) began releasing turkeys for hunting
purposes. Since then the CDFW has released several thousands of turkeys throughout California,
but many of these introduction attempts failed (Gardner et al. 2004). However, the CDFW noticed
an increase in wild turkey population size and range in the 1990s. As a response, they stopped
releasing turkeys in 1999 (Gardner et al. 2004). The reasons behind the recent spread and growth
of wild turkey populations in California are not well understood, but past studies of wild turkeys
in their native range provide knowledge about what effects their population growth.
Nesting failure and food availability are two main factors that determine wild turkey
population growth. These two factors largely depend on habitat type (Spohr et al. 2004; Lehman
et al. 2008; Fuller et al. 2013). The primary reason for nest failure is from predation and the chance
of nest predation depends on the density of vegetation. In areas with somewhat dense vegetation,
turkeys are able to hide their nests from predators well and therefore reduce nest predation. Open
grasslands would have a high amount of predation due to little nest obscurity. However, if the

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vegetation is too dense, such as a dense shrubland, it reduces the ability of a turkey to flee if a
predator arrives, thereby increasing predation (Keegan and Crawford 1997; Lehman et al. 2008;
Fuller et al. 2013). Urban and agricultural areas have been shown to increase food availability.
Wild turkeys are often reliant on agricultural grains and human feeding to maintain large
populations when other food sources are low (Burger 1954; Barrett and Kucera 2005). Although
urban areas may provide additional food, habitat fragmentation from urbanization has resulted in
increased nest predation (Hogrefe et al. 1998; Sphor et al. 2004). Contrastingly, habitat
fragmentation between forests, open areas and agriculture supports large turkey populations
(Glennon and Porter 1999). A majority of these studies on factors affecting population growth took
place in several states in the wild turkey’s native range, but not in California. There have only been
four studies (Burger 1954; Gardner et al. 2004; Barret and Kucera 2005; Wengert et al. 2009) on
wild turkeys in California and three of the four studies are outdated by at least a decade.
Land use changes increasing habitat fragmentation, urbanization and agriculture bring wild
turkeys closer to humans and make them more likely to become pests. Wild turkeys were shown
to damage twenty-three crops, with corn being the most often damaged crop (Tefft et al. 2005).
Additionally, residents of suburban areas have complained about large numbers of turkeys being
a nuisance on their property (Barrett and Kucera 2005). However, no study has analyzed whether
these land use types have encroached on wild turkey habitat or whether wild turkeys have moved
from their original release sites to these land use types. Besides anecdotal and observational
information, there has not been any studies looking at how the population and range of wild turkeys
has changed in California and the possible factors driving the changes.
To better understand wild turkeys in California and as a step towards determining if they
are pests or invasive species, this study maps and analyzes the changes in population and range to
explain the reasons behind the recent spread and growth of wild turkey populations. Climate and
human involvement, such as hunting and translocation, have often influenced population changes
of species, but I hypothesize that land cover, specifically agriculture and urbanization, had the
greatest influence in increasing wild turkey populations in California. To see whether wild turkey
populations are actually increasing, I quantified how wild turkey population changed and mapped
how range changed in California from 1968 to 2012. To determine what factors could have led to
the changes in population, I ran a model to determine the influence of land cover, climate, and
human involvement on wild turkey population changes.

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METHODS
Study system
The sites I looked at were predetermined by the USGS Patuxent Wildlife Research Center’s
North American Breeding Bird Survey that carries out bird surveys to determine presence and
count of many bird species, one of which being the wild turkey. Wild turkeys are generalists; they
can live in many different types of habitats, such as forests, shrublands, grasslands, woodlands and
around agriculture and urban areas (Dickson 1992). Each study site is surrounded by a combination
of these land types. Percentages of each land type: grassland, shrubland, forest, agriculture, and
urban, are given for each site with wild turkeys present in Appendix A.
Data sources
I obtained georeferenced wild turkey population data from the North American Breeding
Bird Survey (BBS) to determine population change. The BBS conducts point count surveys along
40 km long predetermined, non-random roadsides called routes (Figure 1). Citizens skilled in avian
identification conduct surveys between mid-May and early July. Beginning a half hour before
sunrise, the surveys, conducted by one observer, take 4 to 5 hours to complete. Point counts are
taken every 0.8 km along the route, for a total of 50 stops. The point count is conducted by
recording every bird seen or heard within a 400 m radius over a 3 minute timespan (Link and
Sauer 2002; Sauer and Link 2011). The USGS Patuxent Wildlife Research Center has been
conducting the BBS since 1966, but surveys in California began in 1968. I only included data
beginning in 1972 because I originally planned on using LANDSAT satellite imagery that began
in 1972 for land cover type, but the classification was too inaccurate to use. Only one detection of
a wild turkey in 1969 was left out from my analysis. Although the BBS is conducted annually, not
every route is surveyed each year. Turkeys have been observed in 80 routes in California between
1972 and 2013. I only included 76 routes because there was no geospatial information on the path
of these four routes.

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Figure 1: Routes with and without wild turkeys present
I obtained wild turkey translocation and hunting data in California from the California
Department of Fish and Wildlife (CDFW). Translocation data included translocation within
California and between California and another state. Translocation data is from 1959 to 1999 with
the year 1998 missing. For hunting data, CDFW calculated the amount of turkeys hunted by
extrapolating the results from surveys. Mail survey forms were sent out to randomly selected
hunters until approximately 4% of the hunters returned the survey. CDFW has not documented the
specific methodology of the extrapolation. Data spans from 1949 to 2010, with the year 2009
missing. However, there was still hunting in 2009 and after 2010. Instead of leaving zero values
indicating no hunting, which would be incorrect, I used the same data from 2010 for years 2009 to
2013. This was reasonable given that the counties where wild turkeys were hunted had very little
variation since 2000. Both of these datasets are only specific to county, but routes are at a smaller
scale and sometimes span two counties. Therefore it would be incorrect to directly assign each
route a specific value for this data. To account for this inaccuracy, I only included hunting and
translocation as binary data to show whether it occurred in the county the route in. I assigned a

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county to each route based on which county the majority of the route was located in. For a few
routes that were about evenly split between counties, I combined both of the county’s data.
To obtain land cover data, I used USGS National Land Cover Database (NLCD) for 1992,
2001, 2006 and 2011. I only looked at five land cover classes: forest, urban, grassland, agriculture
and shrubland. Because these datasets contain more than just five land cover classes, I combined
similar land cover classes together. For example, I included both evergreen forest, mixed forest,
and deciduous forest under the forest land cover type. I calculated percentage of land cover within
three buffer distances from the route path: 1, 5 and 10 km. Wild turkey flock home range varies
from 1.5 to 10 km2
, therefore I will use multiple distances to account for the various home range
sizes as well as to see if habitat has different effects at different scales (Zeiner et al. 1990; Gardner
et al. 2004; Barrett and Kucera 2005). For years that didn’t have a given NLCD, I used the same
land cover values from the next available dataset with the assumption that the land cover didn’t
change very much between these years. Years prior to and including 1992 had land cover values
from the 1992 dataset, years 1993 to 2001 had values from the 2001 dataset, years 2002 to 2006
had values from the 2006 dataset and years 2007 to 2013 had values from the 2011 dataset.
For climate data, I used temperature and precipitation raster images from Oregon State
University PRISM Climate Group. To obtain average temperature and precipitation for each route,
I used ArcGIS v10.2 (ERSI 2014) to average the temperature and precipitation within 10 km from
each route path. I only used one buffer distance for climate variables because it is unlikely that the
climate will vary significantly under 10km from the route. I squared the average temperature and
precipitation because animals tend to have a quadratic relationship to temperature and precipitation
rather than linear.
Data Analysis
To look at range change, I used kriging, an interpolation technique, in ArcGIS v10.2 to
determine wild turkeys range in California for 6 year intervals. To quantify population change of
wild turkeys at each route and California overall from 1972 to 2013, I used a generalized linear
model (GLM) in R (R Core Team 2014) v3.1.2 using the glm package. I did not run a GLM on
routes that only contained one non-zero data point and I excluded the results of routes where the
generalized linear model didn’t converge due to lack of data. To see the effect of different

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predictors on population change, I used a zero-inflated poisson mixed model in R v3.1.2 using the
glmmADMB package on all 77 routes. I included the percentage of land cover type, temperature,
precipitation, the occurrence of wild turkeys translocation and hunting as covariates. Route was
included as a random effect to account for differences between sites that cannot be explained by
the covariates. I standardized all values, excluding binary and percent values, by subtracting the
mean and dividing by the standard deviation to get more comparable covariate estimates. I ran a
separate zero-inflated model for each of the 3 land cover buffer distances.
RESULTS
Population and Range Change
Wild turkeys throughout California increased in population 10.09% (±0.279%) from 1972
to 2013, according to the generalized linear model (p < 0.0001). Different areas in California
experienced various amounts of population change (Figure 2). Majority of routes experienced less
than a 25% increase in population. More specifically, 31 out of the 61 routes included for this
analysis had an increase between 9% and 20% Route 416 near Meadow Valley in Plumas county
experienced the greatest population change of 108% per year, but was statistically insignificant (p
= 0.0644). Route 210 in Mendocino County and route 12 in Sutter County had increases of 88%
and 67%, respectively, but were also not statistically significant. Route 172, located east of
Berkeley in Contra Costa County, had the greatest statistically significant increase of 39% per year
(p < 0.001). Route 422 located south of Yosemite National Park had the next highest significant
increase of 30% per year (p < 0.01). Route 415 near Crescent Mills in Plumas County and route
202 in Sonoma County had a significant increase of about 28% and 27%, respectively (p < 0.5).
Four routes had a decrease in wild turkey population, but only one route had a significant decrease
in population. This route, route 409 located between Burnt Ranch and Hyampom in Trinity
County, decreased in population of 81% per year (p < 0.05). The percent population change per
year, standard errors and p-values for each route is given in Appendix B.
Range expansion can be seen through a significant (p < 0.0001) increase of 0.266%
(±0.035%) per year in the proportion of wild turkey detected on BBS routes from 1969 to 2013
(Figure 3). This is further supported by kriging maps of six year intervals which show wild turkey

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range, indicated in blue, to be increasing (Figure 4). Based on the maps, the largest expansion was
between 1984 to 1989 interval and the 1990 to 1995 interval. After 1990, the population, indicated
by the darkness of blue, increased more than the range changed. From 1972 to 1989, turkeys
increased 24.8% (±4.56%) per year. However, this increase was actually insignificant (p = 0.489).
In the following period, 1990 to 2013, turkey populations increased by 91.3% (±4.56%) per year
(p < 0.0001). Given that there was no significant increase in population before 1990 and that the
kriging maps show very little change in population and range, I decided to only include years 1990
to 2013 when running the zero-inflated poisson model. Additionally, because the land cover data
only begins in 1992 and all years prior to 1992 would have the same land cover value, the result
of the model would be more accurate after removing the years before 1990.
Figure 2: Geographic distribution of population change. Percent population change, calculated by the generalized
linear model, of wild turkeys from 1972 to 2013. Routes with significant population change are indicated by a black
dot.

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Figure 3: Wild turkey presence. Proportion of Breeding Bird Survey routes in California where wild turkeys were
detected from 1972 to 2013.
Figure 4: Wild turkey range expansion. I mapped the change in wild turkey range and population from 1972 to
2013 by averaging BBS counts for six year intervals for each route and then used kriging (in ArcGIS).
0
0.1
0.2
0.3
0.4
0.5
0.6
1971 1977 1983 1989 1995 2001 2007 2013
Percentage
Year

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Predictors of population change
According to the zero-inflated poisson mixed model for years 1990 to 2013, the most
significant predictor of wild turkey population change for all buffer distances was hunting (p <
0.01) (Table 1). The estimate for hunting was 0.8 for 1 km buffers, 0.74 for 5 km and 0.73 for 10
km. Year was the second most significant predictor than hunting, but still very influential with
estimates of 0.74 for 1 km, 0.78 for 5 km and 0.75 for 10 km. All other factors were much less
influential predictors with estimates of less than 0.1. Precipitation, temperature, translocation and
grassland were not significant for any of the buffer distances. The 95% confidence intervals
included zero, making the effect these covariates neither positive nor negative.
Different factors were significant and had various influences on population change at each
buffer distances. For the 1 km buffer, the percent of urban land cover was significant (p < 0.01)
with an estimate of 0.096. This indicates that areas with increasing urban land cover will
experience increasing population sizes, given that all other factors are held constant. Similarly, for
the 10 km buffer, urban land cover was also significant (p < 0.05) and positively related to
population change (estimate of 0.047). For the 5 km buffer, forest, shrubland and agricultural land
cover were significant covariates, but urban land cover was not (p = 0.078). Forest, shrubland and
agriculture were all inversely related to population change. Forest, with an estimate of -0.077 was
more of an influential predictor than shrubland and agriculture, both with an estimate of -0.023. A
negative estimate indicates that areas that are decreasing with this land cover type have an
increasing wild turkey population.
Table 1: Parameter values for the 3 buffer distances. Summary of each parameter’s influence on wild turkey
populations according to the zero-inflated mixed model. Significant p-values are indicated with *.
Parameter Estimate Standard Error 95% CI p-value
1 km buffer
Intercept -4.0925 2.6214 -9.2303 - 1.0453 0.1185
Year 0.73708 0.06707 0.60563 - 0.86852 <2e-16 *
Precipitation 0.01506 0.04268 -0.06859 - 0.09872 0.7242

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Temperature -0.04387 0.11007 -0.25961 - 0.17186 0.6902
Translocation -0.05994 0.1417 -0.33767 - 0.21778 0.6723
Hunting 0.79648 0.2655 0.27611 - 1.3169 0.0027 *
Urban 0.09639 0.03484 0.02809 - 0.16468 0.0057 *
Forest 0.01566 0.02663 -0.03653 - 0.06785 0.5564
Grassland 0.03881 0.02813 -0.01633 - 0.09395 0.1677
Shrubland 0.03149 0.02788 -0.02315 - 0.08614 0.2587
Agriculture 0.00765 0.02801 -0.04726 - 0.06255 0.7849
Random effect: Variance Standard Deviation
Route 1.323 1.15
AIC: 3562.2
5 km buffer
Intercept 0.95248 0.86066 -0.73438 - 2.6393 0.26843
Year 0.78086 0.0689 0.64582 - 0.91589 <2e-16 *
Precipitation 0.01047 0.04474 -0.07722 - 0.09816 0.81499
Temperature -0.0856 0.10986 -0.30092 - 0.12972 0.43586
Translocation -0.02073 0.14272 -0.30045 - 0.259 0.88454
Hunting 0.84435 0.26364 0.32763 - 1.3611 0.00136 *
Urban -0.0276 0.01568 -0.05833 - 0.00313 0.07837
Forest -0.07653 0.02494 -0.12542 - -0.02764 0.00216 *
Grassland -0.01645 0.01087 -0.03775 - 0.00485 0.13005
Shrubland -0.02385 0.00717 -0.0379 - -0.00981 0.00087 *
Agriculture -0.02371 0.00914 -0.04162 - -0.00581 0.00945 *
Route 1.569 1.253
AIC: 3576
10 km buffer
Intercept -1.45385 0.95128 -3.3183 - 0.41062 0.1264
Year 0.75024 0.06791 0.61713 - 0.88334 <2e-16 *
Precipitation 0.02071 0.04243 -0.06245 - 0.10387 0.6255
Temperature 0.01947 0.11131 -0.1987 - 0.23763 0.8611
Translocation -0.02511 0.14204 -0.30351 - 0.25328 0.8597
Hunting 0.83607 0.27729 0.29259 - 1.3795 0.0026 *

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Urban 0.04671 0.01972 0.00807 - 0.08536 0.0178 *
Forest -0.00489 0.01065 -0.02576 - 0.01598 0.6461
Grassland 0.01008 0.01215 -0.01373 - 0.0339 0.4067
Shrubland 0.00394 0.01232 -0.0202 - 0.02808 0.7489
Agriculture -0.02697 0.01459 -0.05555 - 0.00162 0.0645
Route 1.288 1.135
AIC: 3753
Land Cover
Most wild turkeys were found on routes with majority of forest land cover type. Second
was grassland and then shrubland. For 1 km and 10 km buffers, wild turkeys were common in
land cover types in that same order, but for 5 km the routes with more turkeys tended to be
majority grassland, instead of forest, and followed by shrubland instead of grassland. To get a
perspective of how land cover is changing and predict how wild turkeys should respond, I used
linear regression to calculate the overall change in each land cover type at each buffer distance.
Urban and shrubland both significantly increased for all buffer distances. Forest and grassland
significantly decreased for only the 5 km buffer. Agriculture had no significant changes, but
showed decreases in land cover except for the 10 km, which showed a very small increase
(0.002). All the estimates for agriculture were very small, compared to forest, urban, grassland
and shrub land cover types changes that ranged from 20% to over 40%.

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Figure 5: Wild turkey habitat preference. The number of turkeys found in each land cover type based on the route’s
majority land cover type.
Table 2: Change in land cover. Using linear regression, I calculated the change for each land cover type at each
buffer distance using values from 1992, 2001, 2006 and 2011. Significant p-values are indicated with *.
Land Cover Buffer
Distance
Estimate Std. Error p-value
Urban 1 km 0.19199 0.06022 0.00158 *
5 km 0.23114 0.04801 2.29E-06 *
10 km 0.31237 0.05497 3.02E-08 *
Shrub 1 km 0.4097 0.1218 0.000864 *
5 km 0.4367 0.1457 0.00294 *
10 km 0.3173 0.1274 0.0133 *
Forest 1 km -0.428 0.2323 0.0664
5 km -0.20107 0.09273 0.0309 *
10 km -0.4189 0.2141 0.0513
Grassland 1 km -0.2172 0.1568 0.167
5 km -0.4267 0.1571 0.0069 *
10 km -0.2093 0.1397 0.135
Agriculture 1 km -0.0079 0.149641 0.958
5 km -0.0354 -0.278 0.781
10 km 0.002 0.121 0.988
0
200
400
600
800
1000
1200
1400
Forest Grassland Shrub Urban Agriculture
NumberofTurkeys
Land Cover Type
1 km
5 km
10 km

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DISCUSSION
Wild turkeys have significantly increased in population by 10% per year since 1972, which
is consistent with anecdotes and observations of turkey increases. As hypothesized, urban land
cover had a significant factor influencing population change, and climate and translocation were
not significant. Hunting, which I did not hypothesize to be significant, was the most influential
factor. Additionally, I expected there to be similar results at each buffer distance or if there was a
difference, it would be the 10 km distance because it could be too large of an area to accurately
predict turkey population change. Although I expected agriculture to be significant, it was
surprising that agriculture, forest and grassland were all inversely related to population change,
but only at the 5 km distance. Urban land cover was only significant at 1 and 10 km distances.
These differences likely indicate that habitat effects wild turkeys differently at the local and
landscape scale. Given that the results of the model indicate year as the second most influential
factor, the factors I included in the model are probably not enough to explain why wild turkey
populations have increased.
Population and Range Change
Wild turkey population increase is not centralized in one specific region, but the rate of
increase is similar throughout California. Besides Plumas County, all routes with over 20%
increase in population were in different counties. The fairly large increase of 28% per year for
route 202 in Sonoma County is consistent with Barret and Kucera's research in Annadel State park
located less than 10 km from the route (2005). The wild turkey populations grew rapidly in Sonoma
because there were no limiting factors for reproduction, such as lack of food. The food supply was
actually increased due to humans intentionally and unintentionally feeding turkeys high quality
commercial feed. High food quality can then lead to increased reproduction. With a high nutrition
diet, female turkeys tend to lay more eggs and lay eggs earlier in the breeding season, allowing for
attempts at nesting if one fails (Dickson 1992). Adding to that, wild turkeys already have a large
reproductive capacity. They start reproducing at the age of one and each year they can produce a
clutch size, the number of eggs laid in a single nesting, between 10 and 12. And if their first nesting
fails, they often nest a second time (Lehmen et al. 2008). Route 172, with the largest significant

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increase of 39% per year, had the largest amount of urban area for the 5 and 10 km distances and
fourth largest amount at 1 km. Route 172 probably had similar reasons for population increase as
route 202 in Sonoma county. The large amount of urban area increases the likelihood of wild
turkeys getting additional food from humans.
All routes increased in urban land cover. However, urban land cover did not seem to
increase by such a large percent where it seemed to be encroaching on natural turkey habitat. Wild
turkeys were probably either introduced near urban areas or spread there due to the availability of
increased food supply and quality. Therefore, it is more likely that turkeys moved into areas with
humans, rather than humans moving into areas with turkeys. Surprisingly, many turkeys are not
found in routes with a majority of agriculture or urban land cover, especially considering urban
land cover was a significant influence on population change (Figure 5). This is probably because
CDFW didn’t introduce them into areas that are majority urban or agriculture, but some turkeys
spread to these locations.
Route 409, which declined in population, also increased in urban land cover and was not
very different from other routes in terms of land cover composition and change. I suspect that the
change at this route is not representative of the change in that area. However, routes in that area
did have much smaller population changes of less than 5% increase per year. This is likely do to
later introductions by CDFW in this area according to translocation data (Gardner et al. 2004).
Insignificant results and non-convergent GLMs from calculating population change at each
route is probably due to lack of data. Not every route was surveyed every year and some routes
were surveyed as few as 8 to 10 years. Although the results were insignificant or the GLMS did
not converge, the trends they show, usually of an increasing population, are probably close to what
is actually happening. This is likely because translocation and hunting data often indicated turkeys
in counties that the BBS did not detect them. The BBS data is just lacking data to statistically show
the trend and is probably underestimating wild turkey populations. Wild turkeys probably also
have a larger range than what is shown in kriging maps based of BBS data (Figure 4).
Wild turkeys are found more frequently on routes in northern California, with the exception
of route 106 in southern California near San Diego. The majority of routes where turkeys were
found are primarily in the foothills and mountains of California. This distribution resembles the
distribution of CDFW’s wild turkey release sites from 1959 to 1999 (Gardner et al. 2004). This
makes it unclear whether wild turkeys are found in this pattern because CDFW released them in

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these locations, turkeys have a particular habitat preference, or both. CDFW may have also
released them there because studies suggested that these areas are suitable habitat. Consistent with
studies of wild turkey habitat preference, turkeys seem to be found in places that are primarily
forest, followed by grassland, shrubland or a mixture of these three (Figure 5) (Dickson 1992;
Gardner et al. 2004). This consistency shows that wild turkeys have similar preferences and
characteristics regardless of the specific location they are studied. This allows us to reliably make
inferences about wild turkeys in other states they have not been studied in.
Predictors of population change
Hunting is the most influential predictor of wild turkey populations, with coefficient values
ranging from 0.79 to 0.84. This positive value indicates that wild turkey populations increased
with more hunting. However, it is more likely that relationship is reversed. From looking at the
data, the amount of hunting seemed to increase as a response to an increasing wild turkey
population. The years that turkeys show up in counties and the year hunting starts in those counties
often coincides. For example, in San Diego county turkeys did not show up until 1990 to 1995 and
similarly hunting did not start until 1992. This was not a result of hunting data being inputted as
occurrence data in binary rather than number of turkeys actually hunted, which could show an
increase in counties hunted but not an increase in numbers hunted. However, the amount of turkeys
hunted in California usually increased every year according to CDFW Game Take Survey Reports.
Therefore, this result is actually showing that although hunting is increasing, wild turkey
populations are increasing at an even greater rate. The implications of this could be huge,
especially if CDFW expects wild turkey populations to stay under control or stop increasing from
hunting alone.
As predicted, land cover type was an influential predictor of wild turkey population change.
The percent of urban area was the only significant land cover factor at both the 1 and 10 km
distances. In Sonoma County, urbanization is probably increasing food supply. Additional food
sources support larger populations than a habitat would naturally allow (Burger 1954; Barrett and
Kucera 2005; Glennon and Porter 1999). In winter and times when vegetation does not grow well,
food is scarce, but if wild turkeys are not reliant on natural food sources, the food scarcity won’t
have any effect on them. Contradictorily, urbanization may increase predation for wild turkeys due

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to the habitat fragmentation it creates and therefore reduce populations (Hogrefe et al. 1998; Sphor
et al. 2004). It is likely that this is not a huge influence in many places in California because turkey
predator populations are probably low.
Looking at a 5 km scale, forest had a negative influence on population change. Forest is
one of the wild turkey's primary habitat and also is the most preferred habitat in California,
therefore it seems unreasonable that wild turkeys would increase due to a reduction in forest cover
(Dickson 1992) (Figure 5). Instead, given that forests are decreasing in general, it is more likely
that the negative coefficient could be a coincidental result of the two trends occurring at the same
time (Table 2). Similar to the result with hunting, although decreasing forest cover is correlated
with an increasing population, one is not the cause of another. This suggests that another factor
could be influencing wild turkey population increase. The same could be said of agriculture which
decreased overall as well. For shrubland, however, the reason for the inverse relationship with
population change could be because dense shrubland is bad habitat for turkeys because it can
increase predation as well as limits the mobility of them (Keegan and Crawford 1997; Gardner et
al. 2004; Lehman et al. 2008; Fuller et al. 2013). But this is uncertain given that a large amount of
turkeys resides on routes with majority shrubland (Figure 5).
The effects of land cover type at different scales are surprisingly different and
contradictory. The reasons that different factors are significant at different scales is not clear.
Looking at the land cover, it seems these results are attributed to land cover percent values because
the 1 km and 10 km distances often have very similar values that are different from the 5 km
distance. These results show that wild turkeys are effected by habitat at different scales, both local
and landscape. The different results could be also indicating that wild turkeys have a specific home
range size, making two of the three distances inaccurate. If turkeys have a particular home range,
one would expect the results for that distance to best explain the population change. However, the
home range of wild turkeys specifically in California is unknown, therefore it is hard to say which
of the three distances is probably the most accurate. Also, based on other studies, the home range
has a huge range from 1.5 km2
to 10 km2
scales (Zeiner et al. 1990; Gardner et al. 2004; Barrett
and Kucera 2005). Additionally, it is complicated to compare wild turkeys response to habitat at
different scales because turkeys have a huge capacity for range expansion. Hens disperse up to 13
kilometers to find a nest site and individuals can fly up to 64 kilometers in a week (Barrett and
Kucera 2005). Habitat fragmentation and edge effects, which were not taken into account in my

18
model may also be a cause of these odd results. Looking at the land cover at each routes, many of
them did have similar amounts of several land cover types, often about 20%. Wild turkeys are
often associated with edges and fragmentation and can have large populations in these landscapes
(Glennon and Porter 1999).
Temperature and precipitation did not significantly influence population change. It is
reasonable that temperature would not be influential because wild turkeys are habitat generalists.
In addition, they have a large range that spans the entire United States and successfully survive in
various climates (Dickson 1992). Increased precipitation, on the other hand, has been associated
with lower populations due to increased predation. Predation increases in wet weather because the
wild turkey’s scent is more detectable under wet conditions (Palmer et al. 1993, Roberts et al.
1995, Roberts and Porter 1998). However, other studies have shown that the effect of precipitation
is more complex (Lehman et al. 2008; Fuller et al. 2013). In addition to the strength of their scent,
the probability of predation also depends on the type and density of vegetation cover. A large
amount of precipitation over at least a short duration, such as a few days, can increase vegetation
density and therefore reduce predation because more vegetation cover decreases detectability from
predators. The effect of precipitation is varied such that it could either increase or decrease
populations depending on the vegetation type in the area and the amount and duration of
precipitation. The possibility of both positive and negative effects could be a reason why the 95%
confidence interval for precipitation spanned zero, indicating neither a clear positive nor negative
influence on population change. Consequently, precipitation was not a significant predictor of
population change.
Limitations
The biggest limitation of this study is that it relies solely on the accuracy and consistency
of secondary data. Breeding Bird Survey data has a specific methodology and is much more
consistent than other citizen survey data, such as the Christmas Bird Count. However, it still
contains inconsistencies, most notably that not every route is surveyed every year. There is also
surveyor error due to their varying experience in observing and identifying birds. In addition, the
route locations I used for mapping and obtaining land cover and climate data have low accuracy.
According to BBS, if part of the route was not on the map they were digitizing they estimated the

19
route’s location. Start and end points of the routes were also estimated, which often made the
digitized routes longer than the actual routes surveyed. Also, parts of some routes were not mapped
out in the shapefile due to traffic noise or overlap with other routes. The routes BBS uses may also
cause inaccuracies because they follow a road. Road-based surveys are usually biased because
animals are often reluctant to go near a road. Although this hasn’t been shown with wild turkeys,
it has been shown that a turkey flock is less detectable when near roads and populations are
therefore underestimated (Butler et al. 2007). Also, since BBS routes are located on secondary
roads, which are primarily in rural areas, trends in more urban areas are hard to detect (Butcher et
al. 2014).
Additionally, limited number of years for land cover datasets probably lead to less accurate
results. The USGS does not advise the 1992 dataset to be combined with the other datasets due to
different methods in creating it. I still combined the data because the land cover classes had similar
descriptions and did not look too different visually. However, this could still have led to
inaccuracies. For example, urban area seems much smaller for all 1992 years. This could be due
to the different methods used or due to an actual increase in urban area. The only alternative to the
NLCD would have been classifying satellite imagery, but it is not easy when looking at a large
area, such as California, and it may not have been any more accurate.
Future Research
Although vegetation influences the amount of wild turkey predation, the direct effect of
predator population was not taken into account in this study. Adding a predator population as
another covariate to the model in this study will distinguish whether habitat’s effect on population
is due more to its ability to reduce or increase predation or to other factors, such as food
availability.
From this study and the lack of research on wild turkeys in California, the question of
whether an increase in wild turkey populations has a significant ecological impact remains
unanswered. Like many introduced species, large wild turkey populations have the potential to
impact food web dynamics. Invasive and introduced species, including wild turkeys, can influence
native species populations through both apparent and direct competition. Apparent competition
occurs when growing populations cause their predator population to increase leading to a potential

20
decline in population of another species that their predators prey on. Direct competition occurs
when a species has to compete for habitat and food resources with a similar species that shares its
range. Research into other ground-nesting birds, such as the mountain quail and California quail,
would be useful to see if they are experiencing direct or apparent competition from wild turkeys
because they have similar predators and diet.
The maps of wild turkey population growth can also be used to find localized areas for
further research. Areas with the largest population increase can be studied to see if there are any
detrimental effects, either ecological impacts or as pests, because the effects would be most
noticeable in these areas.
Conclusion
The wild turkey has been introduced into every state except Alaska as well as several other
countries, and in general they have been successful in establishing populations. Besides the wild
turkey, there are many species, usually invasive or introduced, that are spreading their range over
huge areas. These species tend to be generalists and often do well in urban or agricultural
environments, such as the wild turkeys and Eurasian collared doves (Fujasaki et al. 2010). As
urbanization and agriculture increases all over the globe, these species have more areas that they
can expand in successfully. These land cover changes are creating homogenous landscapes that
promote non-native species, the loss of native species and therefore less biodiversity (McKinney
2002).
AWKNOWLEDGEMENTS
I would like to greatly thank Steve Beissinger for telling me about the Breeding Bird
Survey and giving me the suggestion to go about answering my thesis questions with a
modelling approach. My thesis would have been very different if I wasn’t able to discuss my
initial research question with him. He also suggested my mentor, Sarah Maclean, who I am very
grateful for helping me with all the little details and problems I ran into, especially discussing
analysis approaches. I would like to acknowledge Reginald Barret, who provided me with
literature about wild turkeys and who to contact in the California Department of Fish and
Wildlife. From the California Department of Fish and Wildlife, I would like to thank Scott

21
Gardner and Levi Sousa for providing data and answering my questions on wild turkey
management. I would also like to Anne Murray for help with the organization and writing of my
thesis. Finally, thank you to Jenny Sholar, Kaela Shiigi and Erik Schmiett for your feedback as
well as the rest of my peers for the support through these semesters.
REFERENCES
Baldwin, R. A., T. P. Salmon, R. H. Schmidt, and R. M. Timm. 2013. Wildlife pests of
California agriculture: Regional variability and subsequent impacts on management. Crop
Protection 46:29–37.
Barrett, R.H. and T.E. Kucera. 2005. The Wild Turkey in Sonoma County State Parks.
California Department of Parks and Recreation.
Burger, G. V. 1954. Wild Turkeys in Central Coastal California. The Condor 56:198-206.
Butcher, A. J., B. A. Collier, N. J. Silvy, and J. A. Roberson. 2014. Spatial and temporal patterns
of range expansion of white-winged doves in the USA from 1979 to 2007. Journal of
Biogeography 41:1947-1956.
Butler, M. J., W. B. Ballard, M. C. Wallace, and S. J. Demaso. 2007. Road-Based Surveys for
Estimating Wild Turkey Density in the Texas Rolling Plains. Journal of Wildlife
Management 71:1646–1653.
Dickson, J. G. 1992. The wild turkey: biology and management. Stackpole Books,
Mechanicsburg, PA.
Duncan, R. P., T. M. Blackburn, and D. Sol. 2003. The ecology of bird introductions. Annual
Review of Ecology, Evolution, and Systematics 34:71-98.
Fujisaki, I., E. V. Pearlstine, and F. J. Mazzotti. 2010. The rapid spread of invasive Eurasian
Collared Doves Streptopelia decaocto in the continental USA follows human-altered
habitats. Ibis 152:622-632.
Fuller, A. K., S. M. Spohr, D. J. Harrison, and F. A. Servello. 2013. Nest survival of wild turkeys
Meleagris gallopavo silvestris in a mixed-use landscape: influences at nest-site and patch
scales. Wildlife Biology 19:138-146.
Gardner, S., T. Blankinship, J. Decker. 2004. Strategic Plan for Wild Turkey Management.
California Department of Fish and Game. Wildlife Programs Branch. Sacramento, CA.
Glennon, M. J., and W. F. Porter. 1999. Using satellite imagery to assess landscape-scale habitat
for wild turkeys. Wildlife Society Bulletin 27:646–653.

22
Hogrefe, T. C., R. H. Yahner, and N. H. Piergallini. 1998. Depredation of artificial ground nests
in a suburban versus a rural landscape. Journal of the Pennsylvania Academy of Science
72:3–6.
Keegan, T. W., and J. A. Crawford. 1997. Brood-rearing habitat use by Rio Grande wild turkeys
in Oregon. Great Basin Naturalist 57:220–230.
Lehman, C. P., M. A. Rumble, L. D. Flake, and D. J. Thompson. 2008. Merriam's turkey nest
survival and factors affecting nest predation by mammals. The Journal of Wildlife
Management 72:1765-1774.
McKinney, M. L. 2006. Urbanization as a major cause of biotic homogenization. Biological
Conservation 127:247–260.
Spohr, S. M., F. A. Servello, D. J. Harrison, and D. W. May. 2004. Survival and reproduction of
female wild turkeys in a suburban environment. Northeastern Naturalist 11:363–374.
Sauer, J. R., and W. A. Link. 2002. A hierarchical analysis of population change with application
to Cerulean Warblers. Ecology 83: 2832–2840.
Sauer, J. R., and W. A. Link. 2011. Analysis of the North American breeding bird survey using
hierarchical models. The Auk 128:87-98.
Tefft, B. C., M. A. Gregonis, and R. E. Eriksen. 2005. Assessment of crop depredation by wild
turkeys in the United States and Ontario, Canada. Wildlife Society Bulletin 33:590-595.
Wengert, G. M., M. W. Gabriel, R. L. Mathis, and T. Hughes. 2009. Food Habits of Wild
Turkeys in National Forests of Northern California and Central Oregon. Western Birds
40:284–291.
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Vol. II. California Department of Fish and Game, Sacramento, California.
APPENDIX A
Appendix A is included separately. Please contact the author to receive it.
APPENDIX B
Table B1: Population change at each individual route. These results were calculated using a GLM for each route.
Route Latitude Longitude Estimate Standard Error P-value Significance
416 39.94937 -121.038 1.0886 0.5887 0.0644
12 39.27543 -121.734 0.881 0.6503 0.175
210 39.16346 -123.227 0.6697 0.4459 0.133

23
172 37.81835 -122.146 0.39204 0.0583 1.87E-11 ***
422 37.46188 -119.794 0.3064 0.105 0.00351 **
415 40.04023 -120.996 0.2834 0.1393 0.042 *
202 38.37951 -122.514 0.27467 0.08571 0.00135 **
34 35.73357 -118.078 0.24659 0.08571 0.00402 **
181 40.20176 -120.624 0.2237 0.04547 8.63E-07 ***
13 39.12397 -120.760 0.212 0.1787 0.235
357 38.35393 -120.680 0.20584 0.08126 0.0113 *
194 37.21153 -122.157 0.19729 0.03155 3.99E-10 ***
138 35.31838 -120.508 0.18429 0.03321 2.88E-08 ***
15 38.97873 -122.240 0.18139 0.03442 1.37E-07 ***
80 39.76334 -122.239 0.17938 0.08392 0.0326 *
114 34.73757 -120.235 0.16884 0.02712 4.76E-10 ***
173 35.92226 -121.064 0.16613 0.05157 0.00128 **
14 38.80359 -123.585 0.16292 0.03754 1.42E-05 ***
190 37.98349 -120.645 0.15338 0.02342 5.78E-01 ***
83 37.98512 -122.591 0.14388 0.01753 2.26E-16 ***
51 32.90395 -116.233 0.14257 0.07376 0.0533
75 40.52807 -124.164 0.1412 0.1082 0.192
106 33.19909 -116.786 0.1323 0.01307 <2e-16 ***
186 38.54987 -122.719 0.12701 0.02487 3.27E-07 ***
183 39.18442 -122.709 0.12407 0.06137 0.0432 *
81 39.16123 -123.582 0.12135 0.01785 1.06E-11 ***
160 39.56649 -123.398 0.12128 0.02138 1.40E-08 ***
184 39.06869 -121.010 0.11862 0.02577 4.16E-06 ***
189 37.25217 -121.677 0.11788 0.01927 9.58E-10 ***
71 38.04535 -122.867 0.1157 0.0292 7.42E-05 ***
153 38.76876 -120.925 0.1089 0.007789 <2e-16 ***
178 40.17327 -123.604 0.10574 0.02104 5.01E-07 ***
179 40.02796 -122.639 0.10572 0.01459 4.23E-13 ***
182 39.68784 -123.485 0.09925 0.01116 <2e-16 ***
159 39.49790 -121.609 0.09821 0.06706 0.143
193 38.34077 -123.048 0.09727 0.0229 2.15E-05 ***
117 36.46643 -118.655 0.09492 0.06132 0.122
5 40.24319 -124.123 0.09425 0.01644 9.96E-09 ***
54 36.63160 -119.305 0.09215 0.05083 0.0698
25 36.76236 -121.175 0.09107 0.04062 0.0249 *
53 36.22517 -120.519 0.0872 0.02509 0.00051 ***
191 37.12012 -119.701 0.08258 0.04579 0.0713
98 38.72022 -121.036 0.07863 0.009435 <2e-16 ***
108 36.45308 -121.696 0.06964 0.01332 1.70E-07 ***

24
161 40.33571 -121.916 0.0684 0.02314 0.00312 **
6 40.41509 -123.962 0.06403 0.03081 0.0377 *
95 38.55626 -120.291 0.06008 0.06699 0.37
341 35.89089 -118.850 0.05952 0.07675 0.438
123 35.63783 -120.855 0.03829 0.01361 0.0049 **
900 41.90242 -122.563 0.02538 0.04168 0.543
420 38.79958 -120.724 0.01042 0.10213 0.919
97 40.51649 -122.382 0.009377 0.037764 0.804
901 41.52092 -122.520 0.001347 0.027552 0.961
99 33.36574 -118.374 -0.1031 0.1129 0.361
411 40.18032 -122.860 -0.1951 0.1696 0.25
419 38.73968 -120.573 -0.2315 0.1984 0.243
409 40.80313 -123.483 -0.8109 0.3727 0.0296 *

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