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Winter Space Partitioning of Woodpeckers and Nuthatches
in Wisconsin
Bree L. Richardson1,*, Jenna A. Cava2, Richard P. Thiel3, and Jason D. Riddle4
Abstract - Woodpeckers and nuthatches are resident species sharing similar year-round
habitat in northeastern North America, but little is known about how these species distribute
themselves within the same wintering area. From 2009 to 2015, we conducted a
mark–recapture study of 7 Downy Woodpeckers, 15 Hairy Woodpeckers, 9 Red-bellied
Woodpeckers, and 39 White-breasted Nuthatches to determine geographical winter
home-range partitioning between and within species. We used multinomial log-linear
models to estimate the likelihood of capturing each species in a particular baited trap
when other species had been caught in the same trap during the same year. Our results
show the presence of each species influenced the likelihood at least 1 other species would
inhabit the same area. Most of these relationships were positive and indicate active sharing
of the same space. However, Hairy Woodpeckers appeared to deter White-breasted
Nuthatches, and Red-bellied Woodpeckers avoided conspecifics. Little evidence of space
partitioning suggests minimal competition occurs during winter months between these
species. Since these species occupy similar habitats, the appearance of one may indicate
suitable habitat influencing the presence of others.
Introduction
Interspecific competition is often regarded as a major organizing force within
avian communities on several organizational levels (Crowell 1962, Diamond
1978). Early studies frequently attributed interspecific competition as the cause of
distribution across a landscape based on observational evidence (Crowell 1961,
Grinnell 1904). For example, one third of avian dispersal was limited by interspecific
competition in the eastern Andes of Peru (Terborgh 1971). In addition,
interspecific competition can directly affect individual space-use behaviors. For
example, warblers in the Mediterranean changed vertical foraging locations in
trees based upon the presence or absence of other warbler species (Cody and Walter
1976). Individual space-use and its relationship to niche partitioning have been a
particularly fruitful area of study (Cunha and Vieira 2004, Martin et al. 2004, and
Willson 1970).
Niche partitioning as a result of interspecific competition often focuses on
individual foraging behavior, but not on how one species may influence the
presence of another (Morrison and With 1987, Willson 1970). Poecile rufescens
(Townsend) (Chestnut-backed Chickadee) and Poecile atricapillus L. (Black-
12599 Paradise Road, Milladore, WI 54454. 2W174N8473 Schneider Drive, Menomonee
Falls, WI 53051. 37167 Deuce Road, Tomah, WI 54660.4Wildlife Ecology and Management
Discipline, College of Natural Resources, University of Wisconsin-Stevens Point, WI
54481. *Corresponding author - breelacey@gmail.com
Manuscript Editor: Susan Smith Pagano
Winter Ecology: Insights form Biology and History
2017 Northeastern Naturalist 24(Special Issue 7):B32–B41
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capped Chickadee) have been found to alter their natural foraging behavior to
mimic each other when foraging in a mixed flock (Krebs 1973). Mixed-flock
foraging frequently occurs during the winter months due to a decrease in food
supply (Austin and Smith 1972, Morse 1970). Despite increased foraging associations
between wintering species, individuals of some permanent residents, such as
woodpeckers and nuthatches, still maintain winter home ranges to varying degrees
of exclusivity to conspecifics and individuals of other species (Grubb and Pravosudov
2008, Jackson and Ouellet 2002, Jackson et al. 2002, Stickel 1965). This
behavior may affect the distribution of these species across landscapes due to geographical
space partitioning. Spatial data from marked individuals are necessary
to determine these home-range–level interactions within and between species, and
this type of data is sparse for many species.
In North America, woodpeckers and nuthatches comprise an often overlooked
and understudied winter community. Picoides pubescens L. (Downy Woodpecker),
Picoides villosus L. (Hairy Woodpecker), Melanerpes carolinus L. (Red-bellied
Woodpecker), and Sitta carolinensis (Latham) (White-breasted Nuthatch) are
common North American backyard birds considered species of least concern by
the IUCN (Grubb and Pravosudov 2008, Jackson and Ouellet 2002, Jackson et al.
2002, Shackelford et al. 2000). These species are often found foraging in similar
landscapes and exhibit seasonal home-range variation, with the largest home range
occupied during the winter (Nilsson 1976, Wiktander et al. 2001, Williams and
Batzli 1979). Some previous research has been conducted on winter conspecific
territorial behaviors and the niche partitioning of foraging space in these species,
particularly in relation to whether they display pair-bonding behaviors throughout
the entire year (Grubb 1982, Jackson et al. 2002, Kellam 2003, Morrison and
With 1987, Peters and Grubb 1983, Stickel 1965, Willson 1970). There also have
been many studies conducted on nest-cavity selection and partitioning during the
breeding season, although most focus on the structural determinants of partitioning
patterns (Sedgwick and Knopf 1990, Stauffer and Best 1982). Gutzwiller and
Anderson (1988) found interspecific relations were not an important determinant
of species co-occurrence across a landscape during the breeding season. However,
we were unable to find a comprehensive study examining the possibility of winter
space partitioning between all co-occurring woodpecker and nuthatch species. Our
objective was to determine the extent Downy Woodpeckers, Hairy Woodpeckers,
Red-bellied Woodpeckers, and White-breasted Nuthatches influence the presence
of each other at particular geographic points during winter. We used capture data
from a network of traps to evaluate whether the 4 study species tended to share the
same geographical wintering space or partition themselves within a forest fragment
in central Wisconsin.
Study Site
Our study site was located at North Bluff, a 259-ha Precambrian forested
rock outcrop rising 62 m within Sandhill Wildlife Area (SWA) near Babcock, WI
(N44°19'6.1", W90°10'50.6"). SWA is 4090 ha consisting of upland forest and
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marsh habitat. The current forests established in 1930 after a forest fire swept
through the region, and have been subjected to timber harvests since 1960. North
Bluff is directly surrounded by a mixture of marsh, openings created by recent clear
cuts (less than 20 years), and fragments of upland forest. The average basal area across the
entire site was 28 m2 ha-1 (range = 11–57), with Quercus and Populus spp. making
up the dominant tree species.
Methods
Each winter between 2009 and 2015, we live-trapped Downy Woodpeckers,
Hairy Woodpeckers, Red-bellied Woodpeckers, and White-breasted Nuthatches
from late January to snowmelt in March. Trapping occurred every Saturday and
Sunday unless it was snowing or temperatures dropped below -18 °C. Our trapping
scheme consisted of 23 suet-baited live traps (Fiske 1968). The traps were positioned
on tree trunks 1.2 m–1.5 m above the ground and were set approximately 160 m apart
Figure 1. Aerial view of North Bluff within Sandhill Wildlife Area, WI, with live-trap sites
indicated by numbered, filled circles.
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in 2 concentric circles, with 14 surrounding the bluf f and 9 circling the top (Fig. 1).
We opened traps at 06:00 and checked them approximately every 3 hours at ~09:00,
~12:00, and ~15:00. We wired traps shut between Saturday 15:00 and Sunday 06:00
to prevent bait loss. The traps were wired open between Sunday 15:00 and Saturday
06:00 to prevent accidental capture. Bait was left in the traps at the end of trapping on
Sundays and was always absent by the next Saturday when trapping resumed. Squirrels
were known to raid traps when left open, making it likely bait was not available
to the birds throughout the entire week. We recorded the date, time block, and trap
location for every captured bird. We recorded species and marked individuals with
sequentially numbered US Geological Survey aluminum bands. For additional details
on the study site and fieldwork, see Cava et al. (2014).
Data analysis
We used multinomial logistic regression models to estimate the likelihood of
capturing an individual of each target species at a particular trap given the prior
capture of individuals of the same species or other target species at the same trap
during the same year. There were 5 potential capture outcomes: no capture, Downy
Woodpecker, Hairy Woodpecker, Red-bellied Woodpecker, or White-breasted Nuthatch.
The reference group for all models was the “no capture” outcome. Our a
priori set of models contained the following explanatory variables: presence of each
study species at a trap during the same year (4 separate variables) and if the trap
was on the edge of an open area to account for changes caused by tree harvest on
the study site between 2010 and 2011 seasons (Table 1). Positive relationships between
species would indicate active sharing of space. Neutral relationships would
indicate neither species associated with nor avoided each other. Negative relationships
would indicate space partitioning. All models were run in Program R version
3.1.3 (R Core Team 2014) using the “multinom()” function from the “nnet” package
Table 1. Initial model set. Variables included presence of Downy Woodpecker (DOWO), Hairy Woodpecker
(HAWO), Red-bellied Woodpecker (RBWO), or White-breasted Nuthatch (WBNU) in the
same trap during the same year and whether the trap was adjacent to an edge within our forested study
site (EDGE). To minimize false positives, we kept the number of models to a manageable number. We
limited our possible combinations to interactions expected based on what we found in the literature.
Model description Model ID No. parameters
DOWO+HAWO+RBWO+WBNU+EDGE 1 5
RBWO+HAWO+WBNU+EDGE 2 4
DOWO+HAWO+RBWO+EDGE 3 4
DOWO+HAWO+RBWO+WBNU 4 4
RBWO+HAWO+WBNU 5 3
DOWO+HAWO+RBWO 6 3
RBWO+HAWO+EDGE 7 3
RBWO+HAWO 8 2
WBNU+EDGE 9 2
WBNU 10 1
EDGE 11 1
INTERCEPT 12 0
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(Ripley and Venables 2015). We ranked models using AIC values and calculated
AIC weights (ωi) to quantify the data’s relative support for each model (Burnham
and Anderson 2002). We evaluated Beta coefficients for models holding at least
10% relative support (ωi > 0.10) for significance using 95% confidence intervals,
which were calculated in Program R using the “confint()” functio n.
Results
We recorded 554 capture events (of 5799 potential events) of 70 individuals: 7
Downy Woodpeckers, 15 Hairy Woodpeckers, 9 Red-bellied Woodpeckers, and 39
White-breasted Nuthatches. The top model was the global model. It held the overwhelming
majority of support (ωi = 0.98) relative to all other models (ωi < 0.02;
Table 2). A trap’s location within the forest or on its edge, and the presence of each
species at a trap within a wintering season affected the likelihood of capture of at
least 1 other species at the same trap within the same year. Trap location influenced
the capture likelihood of 2 species: Downy Woodpeckers and White-breasted Nuthatches
were both less likely to be captured along the edge compared to within
the forest (β = (-)0.84, 95% CI = (-)1.51–(-)0.16; β = (-)0.89, 95% CI = (-)1.27–
(-)0.47; respectively). The likelihood a Downy Woodpecker would be captured at a
particular trap increased if White-breasted Nuthatches and other individual Downy
Woodpeckers were caught in the trap during the same year (β = 0.72, 95% CI =
0.25–1.18; β = 0.51, 95% CI = 0.07–0.95; respectively). The presence of Downy
Woodpeckers at a trap increased the likelihood a Hairy Woodpecker would also
be caught there (β = 0.50, 95% CI =0.12–0.88). Red-bellied Woodpeckers were
less likely to be captured at a trap if another individual Red-bellied Woodpecker
was caught there (β = (-)0.51, 95% CI = (-)0.89–0.12), but more likely to be captured
at a trap where a White-breasted Nuthatch was caught (β = 0.68, 95% CI =
0.29–1.06). The likelihood a White-breasted Nuthatch was captured at a trap was
influenced by all 4 species. The capture of a Hairy Woodpecker decreased the likelihood
of White-breasted Nuthatch capture (β = (-)0.35, 95% CI = (-)0.63–(-)0.07),
but Downy Woodpecker, Red-bellied Woodpecker, and other White-breasted
Table 2. Model selection results. See Table 1 caption for variable abbreviations.
Model description Model ID No. parameters AIC ωi
DOWO+HAWO+RBWO+WBNU+EDGE 1 5 4958.9 0.98
RBWO+HAWO+WBNU+EDGE 2 4 4967.0 0.02
WBNU+EDGE 9 2 4978.7 4.87 x 10-05
DOWO+HAWO+RBWO+WBNU 4 4 4981.0 1.57 x 10-05
RBWO+HAWO+WBNU 5 3 4995.0 1.44 x 10-08
WBNU 10 1 5005.3 8.38 x 10-11
DOWO+HAWO+RBWO+EDGE 3 4 5030.1 3.47 x 10-16
RBWO+HAWO+EDGE 7 3 5045.0 2.03 x 10-19
EDGE 11 1 5069.0 1.23 x 10-24
DOWO+HAWO+RBWO 6 3 5069.1 1.15 x 10-24
RBWO+HAWO 8 2 5096.2 1.54 x 10-30
INTERCEPT 12 0 5115.4 1.01 x 10-34
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Nuthatches increased the likelihood of capture (β = 0.33, 95% CI = 0.06–0.60; β =
0.44, 95% CI = 0.16–0.72; β = 1.18, 95% CI = 0.86–1.50; respectively). We provide
coefficients and 95% confidence intervals for all possible relati onships in Table 3.
Discussion
Of the 16 potential relationships among our 4 study species, 7 were positive, 7
were neutral, and 2 were negative (Fig. 2). These results suggest that in our study
area most co-occurring, wintering woodpecker and nuthatch species did not partition
home-range space geographically. Our study did not investigate space partitioning in
the vertical dimension. While these species certainly forage at different heights and
are known to partition foraging space in the presence of another individual (Grubb
1982, Peters and Grubb 1984, Willson 1970), our baited traps were able to capture
Table 3. Top model coefficients and 95% confidence intervals (in parentheses). Coefficients represent
the change in likelihood of capturing a target species relative to capturing no species, depending on
whether the same or another species was captured at the same trap in the same year, and whether the
trap was located on a forest edge.
Relative likelihood of species capture
Explanatory variables DOWO HAWO RBWO WBNU
DOWO captured same year 0.51 0.50 -0.01 0.33
(0.07–0.95) (0.12–0.88) (-0.41–0.39) (0.06–0.60)
HAWO captured same year 0.32 ( -0.27 0.20 -0.35
-0.12–0.76) (-0.65–0.11) (-0.17–0.57) (-0.63–(-)0.07)
RBWO captured same year -0.07 -0.23 -0.51 0.44
(-0.51–0.38) (-0.62–0.15) (-0.89–(-)0.12) (0.16–0.72)
WBNU captured same year 0.72 0.18 0.68 1.18
(0.25–1.18) (-0.20–0.56) (0.29–1.06) (0.86–1.50)
EDGE -0.84 -0.38 0.01 -0.89
(-1.51–(-)0.16) (-0.94–0.09) (-0.41–0.44) (-1.30–(-)0.47)
(Intercept) -4.69 -3.77 -4.03 -3.88
(-5.19–(-)4.19) (-4.14–(-)3.40) (-4.43–(-)3.64) (-1.30–(-)0.47)
Figure 2. Space-partitioning relationships
between the 4 study species:
Downy Woodpecker (DOWO), Hairy
Woodpecker (HAWO), Red-bellied
Woodpecker (RBWO), and Whitebreasted
Nuthatch (WBNU). A positive
arrow (solid lines) indicates an
increased likelihood, and a negative
arrow (dot-and-dash lines) indicates
a decreased likelihood, of capturing
one species (or conspecific) based on
another being captured in the same
winter season.
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the majority of individuals utilizing a particular geographic area (Cava et al. 2014),
which allowed us to determine where across the forest fragment they resided, if not
whether they partitioned immediate foraging space. All results should therefore be
interpreted to refer to geographical home-range partitioning.
Surprisingly, almost half of the relationships between species wintering in the
forest fragment were positive. Each species actively shared space with others and
some increased the likelihood another would appear in the same trap; thus, these
species must be using the same geographical space. Two pairs of species—Downy
Woodpeckers and White-breasted Nuthatches, and Red-bellied Woodpeckers and
White-breasted Nuthatches—had reciprocal positive relationships. This result suggests
that within each pair of species, it is likely one did not “attract” the other
to the same area per se, but both species preferred to use the same space without
deterring each other. In contrast, a one-way positive relationship between 2 species
indicates incomplete overlap of space use; Hairy Woodpeckers tended to use
most of the same space as Downy Woodpeckers, but Downy Woodpeckers did
not tend to use all the space Hairy Woodpeckers used. Positive relationships also
occurred within 2 species (Downy Woodpeckers and White-breasted Nuthatches)
indicating conspecifics will use the same space. There were 2 negative relationships
that suggest some space partitioning did occur: Red-bellied Woodpeckers
deterred other individuals of the same species from occupying the same trap and
Hairy Woodpeckers appeared to deter White-breasted Nuthatches. Our use of bait
may have influenced some of the relationships observed; however, we find it unlikely
to have dramatically altered individual space-use in our system. Bait was not
available consistently throughout the trapping season nor while birds were establishing
their home ranges. We also observed that individuals tend to consistently
use their own “cluster” of neighboring traps covering only a portion of our study
area throughout a single season and even across years, meaning they had stable
home ranges and were not simply traveling from trap to trap to take advantage of
the supplied food (R.P. Thiel, unpubl. data). Our results also agree with other winter
behavior studies conducted without bait on these species, suggesting our level of
food supplementation was not sufficient to alter behavioral patterns (Grubb 1982,
Kellam 2003, Morrison and With 1987, Stickel 1965, and Willson 1970).
Most of the relationships we observed are in accordance with previous research
conducted on winter behavior of these species. Both Downy Woodpeckers and
White-breasted Nuthatches positively influenced the appearance of a conspecific at
the same trap in our study. This was expected because both species will maintain
pair bonds and forage near their mate and other conspecifics during the winter
(Grubb 1982, Kellam 2003, Willson 1970). Hairy Woodpeckers do not maintain
pair bonds over the winter and have not been observed actively defending winter
home ranges from conspecifics. This lack of territoriality would lead to individuals
passively sharing space where home ranges overlap (Jackson et al. 2002).
Red-bellied Woodpeckers also do not maintain pair bonds year-round, but their
individual winter territorial behavior towards conspecifics can vary (Shackelford
et al. 2000). In Illinois, Stickel (1965) observed pairs from the previous breeding
season completely exclude each other from separate winter home ranges or share
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2017 Vol. 24, Special Issue 7
the same home range but act antagonistically when in each other’s immediate vicinity.
Our results suggest that Red-bellied Woodpeckers in Wisconsin maintain
separate winter home ranges, as it was unlikely to capture more than one individual
at a particular trap within a season.
The reciprocal positive relationship between White-breasted Nuthatches and
Red-bellied Woodpeckers may be due to similarities in preferred foraging substrate;
Willson (1970) observed both species primarily forage on the same parts of trees
during the winter. The individuals on our site may have been sharing space in order
to use the same preferred foraging substrate. The other reciprocal positive relationship
we observed, between White-breasted Nuthatches and Downy Woodpeckers, is
probably not due to similar foraging habits. White-breasted Nuthatches commonly
use trunks and larger branches (Grubb 1982, Willson 1970), while Downy Woodpeckers
prefer smaller branches when available (Peters and Grubb 1983). Both of
these species avoided traps along the edge of our study site where the forest met
open areas, which may have limited these 2 species to the same areas within the
forest interior.
Only 1 species seemed to actively avoid or be displaced by another: Whitebreasted
Nuthatches were less likely to be captured in traps where Hairy
Woodpeckers were captured in the same year. Both species will forage on trunks
and large branches (Grubb and Pravosudov 2008, Jackson et al. 2002, Willson
1970) and could be competing for these foraging substrates if sharing the same
space. Although we were unable to find direct observations of interactions between
these 2 species in previous literature, our results suggest Hairy Woodpeckers are
socially dominant to White-breasted Nuthatches. We recommend further investigation
into the specifics on the mechanisms driving winter home-range partitioning
between Hairy Woodpeckers and White-breasted Nuthatches.
Overall, our results suggest the majority of woodpeckers and nuthatches in this
wintering community do not partition geographical space between and within species.
This lack of space partitioning on the landscape level allows a larger and more
diverse bird community to inhabit relatively small areas of habitat, such as forest
fragments. The ability of several ecologically similar species to coexist in this way
is likely due to space partitioning on a finer scale, as has been observed in foraging
studies (Morrison and With 1987, Peters and Grubb 1983, Willson 1970). Future
studies should combine information of space partitioning and winter home-range
size to determine the actual amount of habitat required to maintain viable population
sizes of diverse wintering bird communities.
Acknowledgments
We thank the staff at Sandhill Wildlife Area, Babcock, WI, and the Wisconsin Department
of Natural Resources, Madison, WI, for use of their facilities and field site. The
University of Wisconsin-Stevens Point Student Chapter of The Wildlife Society and Student
Government Association provided logistical and financial support. We also thank past
project leaders A. Purdy, B. Sadler, B. Winter, K. Witkowski, R. Sheldon, E. Scherer, D.
Harrington, J. Schroeder, W. Krier, D. Fedro, A. Kuehn, and all other student volunteers
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who assisted in data collection. Banding was conducted under permit number 21040 issued
to R.P. Thiel. The University of Wisconsin-Stevens Point Institutional Animal Care
and Use Committee approved trapping, handling, and marking protocols (protocol number
20011.11.12).
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