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Winter Space Partitioning of Woodpeckers and Nuthatches in Wisconsin
Bree L. Richardson, Jenna A. Cava, Richard P. Thiel, and Jason D. Riddle

Northeastern Naturalist,Volume 24, Special Issue 7 (2017): B32–B41

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Northeastern Naturalist B.L. Richardson, J.A. Cava, R.P. Thiel, and J.D. Riddle 2017 32 Vol. 24, Special Issue 7 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 - Manuscript Editor: Susan Smith Pagano Winter Ecology: Insights form Biology and History 2017 Northeastern Naturalist 24(Special Issue 7):B32–B41 Northeastern Naturalist 33 B.L. Richardson, J.A. Cava, R.P. Thiel, and J.D. Riddle 2017 Vol. 24, Special Issue 7 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 Northeastern Naturalist B.L. Richardson, J.A. Cava, R.P. Thiel, and J.D. Riddle 2017 34 Vol. 24, Special Issue 7 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. Northeastern Naturalist 35 B.L. Richardson, J.A. Cava, R.P. Thiel, and J.D. Riddle 2017 Vol. 24, Special Issue 7 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 Northeastern Naturalist B.L. Richardson, J.A. Cava, R.P. Thiel, and J.D. Riddle 2017 36 Vol. 24, Special Issue 7 (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 Northeastern Naturalist 37 B.L. Richardson, J.A. Cava, R.P. Thiel, and J.D. Riddle 2017 Vol. 24, Special Issue 7 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. Northeastern Naturalist B.L. Richardson, J.A. Cava, R.P. Thiel, and J.D. Riddle 2017 38 Vol. 24, Special Issue 7 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 Northeastern Naturalist 39 B.L. Richardson, J.A. Cava, R.P. Thiel, and J.D. Riddle 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 Northeastern Naturalist B.L. Richardson, J.A. Cava, R.P. Thiel, and J.D. Riddle 2017 40 Vol. 24, Special Issue 7 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). Literature Cited Austin, G.T., and E.L. Smith. 1972. Winter foraging ecology of mixed insectivorous bird flocks in oak woodland in southern Arizona. The Condor 74:17–24. Burnham, K.P., and D.R. Anderson. 2002. Model Selection and Multimodel Inference: A Practical Information-Theoretic Approach. 2nd Edition. Springer-Verlag, New York, NY. Cava, J.A., J.D. Riddle, and R.P. Thiel. 2014. Apparent survival of woodpeckers and nuthatches in Wisconsin. Northeastern Naturalist 21:495:505. Cody, M.L., and H. Walter. 1976. 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