2008 NORTHEASTERN NATURALIST 15(4):485–496
Distribution Patterns of Sciurus niger (Eastern Fox
Squirrel) Leaf Nests Within Woodlots Across a
Suburban/Urban Landscape
Carmen M. Salsbury*
Abstract - To determine habitat characteristics that influence Sciurus niger (Eastern
Fox Squirrel) abundance and distribution within a suburban/urban landscape in the
midwestern United States, I documented the density and placement of fox squirrel
leaf nests in 20 woodlots in the Indianapolis metropolitan area, Marion County, IN.
The woodlots varied in size (0.94 to 19.5 ha), approximate age, shape, and degree of
isolation from other woodlots and suitable squirrel habitat in the surrounding area.
Only 8.0% of nests were located in a tree with another nest, and nests were randomly
distributed in all but one woodlot, where they were uniformly dispersed. Nest density
was not significantly related to woodlot size, approximate age, shape, or degree of
isolation. Fox squirrel leaf nests were not found in greater densities along the edge
of each woodlot, contrary to previous reports. My results suggest that the distribution
patterns of fox squirrels within suburban/urban landscapes are similar to patterns
within landscapes fragmented by agriculture.
Introduction
Habitat selection and space-use patterns of animals have long been of
interest to ecologists. For many terrestrial mammals facing habitat loss,
fragmentation, and degradation due to human activities, the details of habitat
selection and space use are of great importance to the persistence and abundance
of populations. In the agriculturally dominated Midwest, a substantial
effort has been made to examine the sensitivity of mammal populations
inhabiting landscapes fragmented by agriculture (Fitzgibbon 1993, Goheen
et al. 2003, Nupp and Swihart 2000, Swihart and Nupp 1998). Less attention
has focused on mammal populations living within suburban/urban habitats.
As urbanization continues to encroach upon and further modify both agricultural
and natural areas, there is a pressing need to examine the habitat
selection and space-use patterns of species living within suburban/urban
landscapes, in addition to examining the ultimate effects of habitat loss and
fragmentation on persistence and abundance of populations.
North American tree squirrels, such as Sciurus niger Linnaeus (Eastern
Fox Squirrel), are prime subjects for research examining the effects of
habitat fragmentation due to urbanization (Koprowski 2005). This species
readily adapts to and lives in and around suburban and urban centers (Steele
and Koprowski 2001). However, the majority of what is known about the
sensitivity and response to habitat fragmentation of fox squirrels is the result
*Department of Biological Sciences, Butler University, Indianapolis, IN 46208;
csalsbur@butler.edu.
486 Northeastern Naturalist Vol. 15, No. 4
of studies conducted in agriculturally fragmented landscapes as opposed to
landscapes fragmented by urban sprawl (McCleery et al. 2007, Salsbury et
al. 2004). The frequency of Eastern Fox Squirrel colonization was positively
affected by woodlot size in one study conducted within an agriculturally
fragmented landscape in west-central Indiana (Goheen et al. 2003). Yet in
a review by Koprowski (2005), density was negatively related to woodlot
size for S. niger. Further, the degree of isolation of woodlots across agricultural
landscapes in west-central Indiana and east-central Illinois appeared to
have little effect on fox squirrel distributions (Goheen et al. 2003, Nupp and
Swihart 2000, Rosenblatt et al. 1999, Swihart and Nupp 1998). The movements
of Eastern Fox Squirrels between woodlots isolated by agricultural
fields in west-central Indiana were, however, restricted to hedgerows, and
movement across the open agricultural matrix was rare (Sheperd and Swihart
1995).
Suburban/urban landscapes may pose unique challenges to sciurids
inhabiting forest fragments therein. Common elements of many suburban/
urban landscapes, such as parking lots, major roadways, and retention ponds,
are uninhabitable by sciurids. These suburban/urban areas contain little
to no food and may pose an increased threat to survival due to exposure to
predators and motorized vehicles (Williamson 1983). In one of the few studies
focused on urban fox squirrel populations, fox squirrels were found to
avoid paved areas within the urban landscape (McCleery et al. 2007). Other
suburban/urban areas may serve as favorable microhabitats for fox squirrels
as they contain bird feeders, horticultural plantings, or mast-producing trees
(Jodice and Humphrey 1992, McComb 1984, Sexton 1990). Further, while
many suburban/urban areas are likely devoid of many natural predators of
sciurids, such as some hawks, owls, snakes, fox, and mustelids, other threats
to survivorship, such as domestic pets and automobiles, may be substantial
within suburban/urban landscapes (Bowers and Breland 1996, Faeth et al.
2005, Shochat 2004). Previous work indicates that squirrels minimize foraging
activity in areas with high densities of domestic cats and dogs (Bowers
and Breland 1996). Given the unique nature of suburban/urban landscapes, it
is unclear whether the effects of forest fragmentation within an agricultural
landscape are applicable to sciurids living in suburban/urban areas.
The objective of this study was to examine how the size, approximate
age, shape, and degree of isolation of woodlots influence the relative abundance
of fox squirrels living within a suburban/urban landscape. It is unclear
whether the matrix of the suburban/urban landscape is more inhospitable
than the matrix of an agricultural landscape. I assumed that some aspects
of the suburban/urban landscape, such as parking lots and major roadways,
would serve as barriers to squirrel movement; therefore, the abundance of
Eastern Fox Squirrels would be negatively related to woodlot isolation. This
prediction is contrary to observations within agricultural landscapes (Goheen
et al. 2003, Rosenblatt et al. 1999). I also predicted that fox squirrel
abundance would be negatively correlated to woodlot size. Because subur2008
C.M. Salsbury 487
ban/urban woodlots are often adjacent to habitats unsuitable to fox squirrels,
such as parking lots and major roadways, I further predicted that fox squirrels
would not prefer to nest near the edge of the suburban/urban woodlots.
Field-site Description
My study was conducted in 20 woodlots located throughout Marion County,
IN from November 2003 to April 2005. Marion County encompasses the metropolitan
area of Indianapolis, IN. The woodlots I surveyed ranged in size from
0.94 to 19.5 ha, and they varied with regard to appoximate age, shape, and degree
of isolation. Using a 1941 aerial photo of Marion County, I determined that
7 of the surveyed woodlots were fully intact in 1941, 7 were non-existent, and 6
were composed of partially wooded areas mixed with open farmland.
Many of the woodlots surveyed were surrounded, in part, by a variety of
different habitats known to be suitable for fox squirrels, such as park woods
and wooded residential areas. Likewise, a number of the woodlots were
surrounded, in part, by habitats unsuitable for Eastern Fox Squirrels, such
as major roads (4 or more traffic lanes) and highways and large commercial
areas (buildings and parking lots; Fig. 1). Although I did not characterize
vegetative characteristics in detail for this study, the woodlots resembled
disturbed woodlots characterized in a previous study (Salsbury et al. 2004)
conducted in the same area. The woodlots in this study consisted of disturbed
secondary growth stands comprised of deciduous trees, most notably
Quercus rubra L. (Northern Red Oak), Q. alba L. (White Oak), Acer saccharum
Marshall (Sugar Maple), A. rubrum L. (Red Maple), Fraxinus spp.
(ash), Ulmus spp. (elm), Carya spp. (hickory), and Celtis occidentalis L.
(Hackberry). Levels of disturbance within the woodlots varied from the presence
of human footpaths to piles of trash and yard waste to felled trees. The
density and composition of the understory and herbaceous layer also varied
Figure 1. Aerial
photo (2004) of
one of the 20
suburban/urban
woodlots in
Marion County,
IN surveyed
for this study.
The woodlot
is bounded by
a polygon and
the leaf nests by
circles therein.
Photo scale
1:3500.
488 Northeastern Naturalist Vol. 15, No. 4
among the woodlots. The understory, when present, generally consisted of
invasive Lonicera maackii (Rupr.) Herder (Amur Honeysuckle) and canopy
tree seedlings. The herbaceous layer often consisted of many native grasses
and ephemeral spring wildflowers as well as invasive species such as Alliaria
petiolata (Bieb.) Carvara and Grande (Garlic Mustard), Euonymous
fortunei (Turcz.) Hand. - Maz. (Winter Creeper), and Rosa multiflora Thunb.
ex Murr. (Multiflora Rose).
Methods
Leaf nest surveys
I estimated the presence and relative abundance of fox squirrels in each
woodlot by counting the number of leaf nests present between November
2003 and April 2005. Previous studies showed that leaf-nest abundance may
be used to estimate tree squirrel population density in an area (Don 1985,
Wauters and Dhondt 1988). I did not attempt to estimate actual densities of
fox squirrels in each woodlot, but instead used nest densities as an indicator
of relative squirrel densities among woodlots. With the help of assistants,
I surveyed each woodlot once during the course of the study to record the
number and location of the fox squirrel leaf nests. We surveyed all woodlots
after leaf fall, when nests were most visible. We located the nests by walking
straight-line transects through each of the woodlots and identifying each
nest with the use of binoculars from the ground level. Once a nest was found,
we geo-referenced the exact location to within 6 m using a handheld GPS
unit (Garmin V), and we marked each nest tree with a dot of flour. I walked
behind each assistant through each woodlot to ensure that we recorded the
location of every active fox squirrel leaf nest. We included only active nests
in this study. We considered a nest to be inactive if we could see daylight
through the nest when viewing it from below or if nest material was hanging
from the central body of the nest.
I was confident that all nests included in this study were Eastern Fox
Squirrel nests, as Eastern Fox Squirrels are the dominant tree squirrel in
central Indiana (Mumford and Whitaker 1982), and I observed many fox
squirrels within the woodlots surveyed. Sciurus carolinensis Gmelin (Eastern
Gray Squirrel), Tamiasciurus hudsonicus Erxleben (Red Squirrel), and
Glaucomys volans L. (Southern Flying Squirrel) are also found throughout
Indiana, and all are known to build or occasionally inhabit leaf nests for
shelter (Edwards et al. 2003, Mumford and Whitaker 1982, Yahner 2003).
However, no individuals of these species were observed within the woodlots
surveyed. Further, Eastern Gray Squirrels are rare in Marion County and
thought to be decreasing in number in the northern half of the state (Goheen
et al. 2003, Mumford and Whitaker 1982). The leaf nests of Red Squirrels
tend to be smaller and more compact than those of Sciurus spp. (Mumford
and Whitaker 1982), and Red Squirrels tend to prefer to nest in conifers
(Yahner 2003), which were not present in the woodlots surveyed. Southern
Flying Squirrels are almost exclusively cavity nesters (Mumford and Whi2008
C.M. Salsbury 489
taker 1982) that are rarely found in small (<4.6 ha) isolated woodlots (Nupp
and Swihart 2000). Further, I could find no accounts of Southern Flying
Squirrels in Marion County, IN (Mumford and Whitaker 1982).
Analysis
All nest locations were plotted on 2004 geo-referenced aerial photographs
of Marion County, IN using ArcGIS software (ESRI version 9.1). A polygon
outlining the boundaries of each woodlot was created in ArcGIS and this
delineation allowed me to calculate the total area of each woodlot. I also set
a 10-m wide internal-edge buffer for each woodlot. Wales (1972) found that
major vegetative changes caused by the edge generally extend 10 to 20 m into
forests depending on exposure, and 10 m appeared to be sufficient to capture
the edge vegetation for woodlots in this study (C.M. Salsbury, pers. observ.).
I calculated the area of each edge-buffer and subtracted this value from the
total area to determine the interior area of each woodlot. Nest densities were
calculated for the total area, the edge buffer, and the interior of each woodlot.
A ratio of edge buffer area to total area (hereafter the “buffer ratio”) was
calculated for each woodlot to serve as a measure of the relative amount of edge
present for each woodlot. I also estimated the fractal dimension (FD; McGarigal
and Marks 1995) of each woodlot as another estimator of woodlot shape.
The FD of each woodlot was calculated as 2 times the logarithm of the woodlot
perimeter (m) divided by the logarithm of the woodlot area (m2).
I estimated the isolation of each woodlot surveyed in the suburban/urban
landscape in two ways. First, I determined the distance between the woodlot
of interest and the nearest woodlot ≥1.0 ha in area. I chose 1.0 ha as the
minimum size, as this area was roughly equivalent to the area of the smallest
woodlot surveyed in this study. I recorded the Euclidian distance (m) in
most cases, unless there were barriers or unsuitable habitats, such as retaining
ponds, large expanses of parking lot, or commercial buildings, across
which squirrels could not move along the straight-line route. Where barriers
occurred, I measured the shortest passable route between the woodlots.
Second, I generated an “isolation index” for each woodlot that incorporated
the suitability of the habitats surrounding each woodlot as potential fox
squirrel habitat. I assigned values to habitat types ranging from 5 (impass-
Table 1. Habitat suitability scores used in the calculation of isolation indices for 20 urban
woodlots surveyed in Marion County, IN from 2003 to 2005. Scores represent the suitability of
habitats adjacent to surveyed woodlots to support fox squirrel populations. The habitat scores
range from 5 for unsuitable habitats to 1 for optimal habitats.
Score Habitat type
5 Bodies of water, commercial buildings and parking lots lacking trees, major highways
4 Commercial areas with few trees, streets with >2 lanes
3 Open green areas lacking trees
2 Tree-lined streets with 2 lanes
1.5 Wooded residential areas
1 Woodlots, wooded parks
490 Northeastern Naturalist Vol. 15, No. 4
able or unsuitable) to 1 (optimal or ideal habitats) (Table 1). I measured the
length (m) of each woodlot perimeter and calculated a weighted average
habitat score using the length of the perimeter corresponding to each of the
adjacent habitat types. This weighted average served as the isolation index.
I calculated the average distance between nests in each woodlot using the
nearest-neighbor-distance spatial analysis tool in ArcGIS Toolbox. I used
the nearest-neighbor index to test whether the nests were clustered, random,
or uniformly distributed within each woodlot.
I calculated coefficients of variation corrected for bias (CV; Sokal and
Rohlf 1981) for woodlot size, shape as represented by buffer ratio, and degree
of isolation as represented by the isolation index and distance to the
nearest woodlot. The effects of woodlot size, shape, and isolation were examined
using a stepwise linear regression analysis. I assigned woodlot area,
buffer ratio (arcsine transformed), FD, isolation index, and nearest-woodlot
distance as independent predictors of total nest density. I set conditions to
enter and exit the model to α = 0.15 and 0.20, respectively. I also compared
nest density in the edge buffer to nest density in the interior for each woodlot
using a paired t-test.
Fox squirrels are known to nest in tree cavities (Baumgartner 1939, Koprowski
1994) and the abundance of tree cavities within woodlots increases
with woodlot age (Newton 1994). I was unable to document the abundance
of tree cavities in the woodlots surveyed in this study. Thus, to rule out the
possibility that leaf nest densities were lower in some woodlots due to higher
tree-cavity availability, I compared the nest densities of the 7 “old” woodlots
(those intact in 1941) with the nest densities of the 7 “young” woodlots
(those non-existent in 1941) using a one-tailed t-test. I tested the a priori
assumption that leaf nest densities would be lower in the older woodlots. I
used Minitab statistical software (Release 13 for Windows) to perform all
statistical analyses, and I assumed statistical significance at α = 0.05.
Results
I located 498 leaf nests in 20 woodlots throughout Marion County, IN.
Among all woodlots combined, I observed 19 trees with more than one nest;
the most nests observed in one tree were 3. Of the 498 nests, only 40 (8.0%)
were found in trees with at least one other nest.
Woodlot characteristics varied among the woodlots surveyed (Table 2).
Of the four variables for which I calculated the coefficient of variation,
distance to the nearest woodlot (CV = 112.32) displayed the most variation,
followed by woodlot size (CV = 81.45), shape as represented by buffer ratio
(CV = 40.98), and degree of isolation as represented by the isolation index
(CV = 31.35). The greatest distance to the nearest woodlot ≥1 ha was 1135
m and the shortest distance was 10 m. The isolation index of the woodlots
varied from 4.93 for the most-isolated woodlot, which was surrounded by
retention ponds, commercial areas, and a six-lane highway, to 1.0 for the
least-isolated woodlot, which was adjacent to another wooded area and sur2008
C.M. Salsbury 491
rounded by wooded residential areas. Most woodlots, however, were only
moderately isolated, and nearly all were adjacent, in some degree, to wooded
residential areas.
Nest density also varied among woodlots (Table 2); however, stepwise
regression analysis indicated that woodlot area, buffer ratio, FD, distance to
nearest woodlot, and isolation index did not significantly explain the variation
in nest density among woodlots. Nest density was negatively related to
woodlot area, and woodlot area was the only variable to enter the stepwise
model; however, the model was not statistically significant (Fig. 2). Nest
density was not significantly higher within the 10-m internal-edge buffer
compared to the interior of each woodlot (paired t = - 0.38, df = 19,
P = 0.710; Table 2). The distribution of the nests varied significantly from
random in only one woodlot and, in this case, the nests were uniformly dispersed.
Nest density also did not differ between “old” and “young” woodlots
(one-tailed t = -1.52, d.f. = 10, P = 0.920).
Discussion
Forest fragmentation within a suburban/urban landscape in the midwestern
United States does not appear to negatively affect Eastern Fox
Squirrel presence or abundance as indicated by leaf-nest density. The
leaf-nest density within woodlots was not influenced by woodlot size, approximate
age, shape, or degree of isolation. This result agrees, in part,
with observations of fox squirrel colonization patterns in forest fragments
across agricultural landscapes. As in the current study, Eastern Fox Squirrel
presence within forest fragments was unaffected by the distance to the
nearest forest patch (Goheen et al. 2003, Nupp and Swihart 2000, Rosenblatt
et al. 1999, Swihart and Nupp 1998). The well-developed dispersal
ability of Eastern Fox Squirrels and their willingness to move across the
landscape were suggested to explain the colonization patterns within an
Table 2. Descriptive statistics of urban woodlot characteristics and fox squirrel leaf-nest densities.
The results are for 20 woodlots surveyed in Marion County, IN from 2003 to 2005.
Factor Mean Range SD
Woodlot area (ha) 5.83 0.94–19.50 4.690
Buffer area (ha) 1.27 0.53–2.90 0.771
Interior area (ha) 4.53 0.34–16.77 4.071
Buffer ratio 0.27 0.11–0.64 0.111
Fractal dimension 1.32 1.24–1.41 0.045
Isolation index 2.63 1.00–4.93 0.813
Distance to nearest woodlot ≥1 ha (m) 279.90 10.00–1135.00 310.500
Total number of nests 24.90 5.00–71.00 18.220
Number of nests in buffer 5.70 1.00–19.00 4.219
Number of nests in interior 19.20 2.00–60.00 16.340
Total nest density (per ha) 5.09 1.30–12.37 3.093
Buffer nest density (per ha) 4.83 0.49–12.00 2.889
Interior nest density (per ha) 5.15 0.63–15.11 3.759
Nearest neighbor distance among nests (m) 26.31 13.90–49.17 9.570
492 Northeastern Naturalist Vol. 15, No. 4
agricultural landscape (Mech and Zollner 2002, Swihart and Nupp 1998,
Zollner 2000). These factors may also explain the current findings. I did
not track individual movements in the current study, but I did observe
fox squirrels frequently moving from woodlots into surrounding park or
residential areas. Further, I observed leaf nests in trees neighboring woodlots
and occasionally observed dead fox squirrels on roadways separating
woodlots from surrounding habitats. These observations, along with the
lack of a relationship between leaf-nest density and woodlot isolation,
suggest that fox squirrels are frequenting wooded parks and residential
areas between woodlots and that these areas may serve as permanent
habitats or dispersal corridors. The fact that fox squirrels were present
within even the most isolated woodlots in this study, at relative densities
similar to non-isolated woodlots, suggests that even restricted access
to wooded residential areas is sufficient to support colonization. These
findings are consistent with results from a previous study of fox squirrel
movement patterns within an urban landscape (McCleery et al. 2007). Although
McCleery et al. found that fox squirrels avoided paved areas, these
areas, ultimately, did not restrict their movements across the landscape.
Whether wooded residential habitats serve as sources or sinks (Pulliam
1988) for fox squirrel populations is unknown. Further investigation of the
population dynamics of fox squirrels living in wooded residential areas is
necessary to gain a complete understanding of the effects of habitat frag-
Figure 2. Relationship between woodlot area and density of leaf nests constructed by
Eastern Fox Squirrels in 20 suburban/urban woodlots surveyed in Marion County,
IN. Regression line and statistics depict the results of a simple linear regression.
2008 C.M. Salsbury 493
mentation on fox squirrels within suburban/urban landscapes.
Although not significant, the tendency for leaf-nest density to be negatively
related to woodlot size is in agreement with previous findings that
show a negative relationship between density and woodlot size (Koprowski
2005, but see Goheen et al. 2003). Nest densities in small woodlots were
either similar to or greater than those in large woodlots in the current study
(Fig. 2). This finding, along with the presence of leaf nests in all woodlots
surveyed, suggests that home-range compaction may have occurred in the
smaller woodlots. A positive relationship between home-range size and
woodlot area has been observed for Eastern Fox Squirrels in previous studies
(Baumgartner 1943, Koprowski 2005, Shepard and Swihart 1995). Further,
the similar nest densities between “old” and “young” woodlots suggests that
woodlot age and, in turn, tree-cavity availability have little influence on leafnest
densities within the suburban/urban landscape.
Leaf-nest distribution within suburban/urban woodlots rarely differed
from random. Salsbury et al. (2004) found that Eastern Fox Squirrels preferred
to nest in trees with diameters at breast height larger than average
in fragmented suburban/urban woodlots, but they showed no consistent
preference for tree species. If Eastern Fox Squirrels observed in this study
placed nests in trees with a larger than average diameter at breast height,
the random nest distribution suggests that these large trees must have been
randomly distributed or highly abundant in all but one woodlot observed.
Further, leaf nests were not more likely to be located near the woodlot edge
than in the woodlot interior. Previous examination of habitat use by fox
squirrels inhabiting agricultural landscapes in Pennsylvania indicates that
they prefer forest edges to the forest interior (Derge and Yahner 2000, Drake
and Brenner 1995). Fox squirrels living within agricultural landscapes may
prefer forest edges because of their close proximity to nearby agricultural
fields where they occasionally feed (Korschgen 1981, Nixon and Hansen
1987). With the exception of neighboring park woods and residential areas,
the matrix surrounding the suburban/urban woodlots surveyed in this study
was most likely devoid of food. However, greater utilization of forest edges
by fox squirrels may be better indicated by activity patterns not observed in
this study rather than by leaf-nest placement.
The results of this study suggest that Eastern Fox Squirrels in the midwestern
United States have adjusted well to the unique nature of the suburban/
urban landscape and the current level of habitat fragmentation.
Eastern Fox Squirrels appear to readily use and move through residential
areas, and paved areas and major roadways seem to pose little deterrent
to squirrel movement. Future research is needed, however, to determine
the importance of residential areas to Eastern Fox Squirrel persistence and
abundance, as squirrel presence in these areas may not be a good indicator
of habitat quality (Van Horne 1983). Further, dispersers move more slowly
through matrix areas devoid of quality food, shelter, and protection from
predation, which in turn reduces dispersal success (Bakker and Van Vuren
494 Northeastern Naturalist Vol. 15, No. 4
2004, Zollner and Lima 2005). Thus, as matrix areas expand across suburban/
urban landscapes, the persistence and abundance of Eastern Fox Squirrels
may be negatively affected by fragmentation, as has been observed for
other tree squirrel species (Swihart and Nupp 1998). Future studies of fox
squirrel metapopulations living within suburban/urban landscapes are necessary
to determine at what point fragmentation begins to negatively influence
persistence and abundance.
Acknowledgments
I thank Rebecca Dolan, Robert Swihart, David Guynn, and three anonymous
reviewers for helpful comments on this manuscript. Data collection was made possible
through the helpful assistance of William Peterman, Jessica Stephens, Hayley
Withers, and Erica Conn. I also thank many public and private landowners for access
to their woodlots. Funding for this research was provided by a Faculty Fellowship
awarded to C.M. Salsbury by the Holcomb Awards Committee at Butler University.
This research was conducted under the support of the Center for Urban Ecology at
Butler University.
Literature Cited
Bakker, V.J., and D. Van Vuren. 2004. Gap-crossing decisions by the Red Squirrel, a
forest-dependent small mammal. Conservation Biology 18:689–697.
Baumgartner, L.L. 1939. Fox Squirrel dens. Journal of Mammalogy 20:456–465.
Baumgartner, L.L. 1943. Fox Squirrels of Ohio. Journal of Wildlife Management
7:193–202.
Bowers, M.A., and B. Breland. 1996. Foraging of Gray Squirrels on an urban-rural
gradient: Use of the GUD to assess anthropogenic impact. Ecological Applications
6:1135–1142.
Derge, K.L., and R.H. Yahner. 2000. Ecology of sympatric Fox Squirrels (Sciurus
niger) and Gray Squirrels (S. carolinensis) at forest-farmland interfaces in Pennsylvania.
American Midland Naturalist 143:355–369.
Don, B.A.C. 1985. The use of drey counts to estimate Grey Squirrel populations.
Journal of Zoology, London 206:282–286.
Drake, J.C., and F.J. Brenner. 1995. Comparison of habitat preferences of Gray and
Fox Squirrels in Northwestern Pennsylvania. Journal of the Pennsylvania Academy
of Science 69:73–76.
Edwards, J., M. Ford, and D. Guynn. 2003. Fox and Gray Squirrels (Sciurus niger
and S. carolinensis). Pp. 248–267, In G.A. Feldhamer, B.C. Thompson, and J.A.
Chapman (Eds.). Wild Mammals of North America: Biology, Management, and
Conservation. The Johns Hopkins University Press, Baltimore, MD. 1216 pp.
Faeth, S.H., P.S. Warren, E. Shochat, and W.A. Marussich. 2005. Trophic dynamics
in urban communities. BioScience 55:399–407.
Fitzgibbon, C.D. 1993. The distribution of Gray Squirrel dreys in farm woodland:
The influence of wood area, isolation, and management. Journal of Applied Ecology
30:736–742.
Goheen, J.R., R.K. Swihart, T.M. Gehring, and M.S. Miller. 2003. Forces structuring
tree squirrel communities in landscapes fragmented by agriculture: Species
differences in perceptions of forest connectivity and carrying capacity. Oikos
2008 C.M. Salsbury 495
102:95–103.
Jodice, P.G.R., and S.R. Humphrey. 1992. Activity and diet of an urban population of
Big Cypress Fox Squirrels. Journal of Wildlife Management 56:685–692.
Koprowski, J.L. 1994. Sciurus niger. Mammalian Species 479:1–9.
Koprowski, J.L. 2005. The response of tree squirrels to fragmentation: A review and
synthesis. Animal Conservation 8:369–376.
Korschgen, L.J. 1981. Foods of Fox and Gray Squirrels in Missouri. Journal of Wildlife
Management 45:260–266.
McCleery, R.A., R.R. Lopez, N.J. Silvy, and S.N. Kahlick. 2007. Habitat use of fox
squirrels in an urban environment. Journal of Wildlife Management 71:1149–
1157.
McComb, W.C. 1984. Managing urban forests to increase or decrease Gray Squirrel
populations. Southern Journal of Applied Forestry 8:31–34.
McGarigal, K., and B.J. Marks. 1995. FRAGSTATS: Spatial pattern analysis program
for quantifying landscape structure. United States Forest Service, Pacific
Northwest Research Station, Portland, OR. General Technical Report PNWGTR-
351.
Mech, S.G., and P.A. Zollner. 2002. Using body size to predict perceptual range.
Oikos 98:47–52.
Mumford, R.E., and J.O. Whitaker, Jr. 1982. Mammals of Indiana. Indiana University
Press, Bloomington, IN. 537 pp.
Newton, I. 1994. The role of nest sites in limiting the numbers of hole-nesting birds:
A review. Biological Conservation 70:265–276.
Nixon, C.M., and L.P. Hansen. 1987. Managing forests to maintain populations of gray
and fox squirrels (Technical Bulletin 5). Illinois Department of Conservation.
Nupp, T.E., and R.K. Swihart. 2000. Landscape-level correlates of small-mammal assemblages
in forest fragments of farmland. Journal of Mammalogy 81:512–526.
Pulliam, H.R. 1988. Sources, sinks, and population regulation. American Naturalist
132:652–661.
Rosenblatt, D.L., E.J. Heske, S.L. Nelson, D.M. Barber, M.A. Miller, and B. MacAllister.
1999. Forest fragments in east-central Illinois: Islands or habitat patches
for mammals? American Midland Naturalist 141:115–123.
Salsbury, C.M., R.W. Dolan, and E.B. Pentzer. 2004. The distribution of Fox Squirrel
(Sciurus niger) leaf nests within forest fragments in Central Indiana. American
Midland Naturalist 151:369–377.
Sexton, O.J. 1990. Replacement of Fox Squirrels by Gray Squirrels in a suburban
habitat. American Midland Naturalist 124:198–205.
Sheperd, B.F., and R.K. Swihart. 1995. Spatial dynamics of Fox Squirrels (Sciurus
niger) in fragmented landscapes. Canadian Journal of Zoology 73:2098–2105.
Shochat, E. 2004. Credit or debit? Resource input changes population dynamics of
city slicker birds. Oikos 106:622–626.
Sokal, R.R., and F.J. Rohlf. 1981. Biometry. W.H. Freeman and Company, New York,
NY. 859 pp.
Steele, M.A., and J.L. Koprowski. 2001. North American Tree Squirrels. Smithsonian
Institution Press, Washington, DC. 201 pp.
Swihart, R.K., and T.E. Nupp. 1998. Modeling population responses of North American
tree squirrels to agriculturally induced fragmentation of forests. Pp. 1–19,
In M.A. Steele, J.F. Merritt, and D.A. Zegers (Eds.). Ecology and Evolutionary
Biology of Tree Squirrels. Special Publication 6, Virginia Museum of Natural
History, Martinsville, VA. 320 pp.
496 Northeastern Naturalist Vol. 15, No. 4
Van Horne, B. 1983. Density as a misleading indicator of habitat quality. Journal of
Wildlife Management 47:893–901.
Wales, B.A. 1972. Vegetation analysis of north and south edges in a mature oakhickory
forest. Ecological Monographs 42:451–471.
Wauters, L.A., and A.A. Dhondt. 1988. The use of Red Squirrel (Sciurus vulgaris)
dreys to estimate population density. Journal of Zoology, London 214:179–187.
Williamson, R.D. 1983. Identification of urban habitat components which affect
Eastern Gray Squirrel abundance. Urban Ecology 7:345–356.
Yahner, R.H. 2003. Pine squirrels (Tamiasciurus hudsonicus and T. douglasii). Pp.
268–275, In G.A. Feldhamer, B.C. Thompson, and J.A. Chapman (Eds.). Wild
Mammals of North America: Biology, Management, and Conservation. The
Johns Hopkins University Press, Baltimore, MD. 1216 pp.
Zollner, P.A. 2000. Comparing the landscape level perceptual abilities of forest sciurids
in fragmented agricultural landscapes. Landscape Ecology 15:523–533.
Zollner, P.A., and S.L. Lima. 2005. Behavioral tradeoffs when dispersing across a
patchy landscape. Oikos 408:219–230.