Date Published: March 20, 2019
Publisher: Public Library of Science
Author(s): Lauren Macfarland, Nancy A. Mahony, Megan Harrison, David Green, Łukasz Kajtoch.
Tree cavities provide a critical resource for cavity-nesting animals, and high quality cavities can be difficult for animals to acquire in habitats where competition is high. We investigated the breeding performance of Lewis’s Woodpeckers in three habitat types in British Columbia, Canada in 2013 and 2014. We also assessed whether the number of nest competitors and cavity availability influenced the habitat specific breeding performance of this threatened cavity nesting species. We found that daily nest survival rate was lower in burned habitat (0.15 ± 0.08 (0.05–0.37)) than in live pine (0.72 ± 0.10 (0.51–0.87)) or cottonwood (0.69 ± 0.09 (0.51–0.83)) habitats. However, hatching success (the proportion of eggs that hatch) was lower in live pine habitat (0.59 ± 0.09 95% CI) than burned (0.77 ± 0.19 95% CI) or cottonwood (0.80 ± 0.07 95% CI) habitat, and the fledging success of successful nests in live pine and burned habitat (1.86 ± 0.31 and 1.88 ± 0.59 95% CI, respectively) was slightly lower than in cottonwood habitat (2.61 ± 0.45 95% CI). Consequently, Lewis’s Woodpeckers in cottonwood habitat produced more fledglings per nesting attempt (2.05 ± 0.49 95% CI) than in live pine (1.53 ± 0.35 95% CI) or burned (0.79 ± 0.49 95% CI) habitat. Habitats differed in the number of nesting competitors and the number of suitable cavities surrounding active Lewis’s Woodpecker nests. Our results showed that cavity density best explained breeding performance differences although the mechanisms remain unclear. There was no evidence that the number of heterospecific nest competitors, including the invasive European Starling (Sturnus vulgaris), explained or influenced Lewis’s Woodpecker breeding performance. Cavity density influenced the productivity of successful nests but did not explain habitat differences in hatching success or daily nest survival. Further work is required to understand the mechanistic basis for the habitat specific breeding performance of Lewis’s Woodpeckers. Habitat differences in breeding performance in British Columbia are not consistent with those in other regions, highlighting the importance of regionally-specific demographic data for managing species at risk.
Tree cavities provide critical breeding habitat for a large number of vertebrates and individuals may increase their fitness by choosing cavities that lead to high reproductive success [1–4]. The quality of cavities affects the reproductive success of many cavity-nesting species because cavity size and entrance height, shape and orientation can all influence the impact of extreme weather events and predation [5–10]. Consequently, cavity availability can be a limiting factor for some populations of mammals , birds , and amphibians . Identifying habitats that support higher densities of high quality cavities, that produce higher breeding success, may therefore be important for supporting populations of cavity-nesters of conservation concern.
Lewis’s Woodpeckers are weak cavity-nesters, prone to using tree cavities created by primary cavity-nesters in cottonwood, live pine and burned habitat within British Columbia, Canada. We found that hatching success and brood size was higher for nests in cottonwood and burned habitat compared to live pine habitat. Daily nest survival was higher for nests in cottonwood and live pine habitat compared to burned habitat, and therefore overall productivity was higher for nests in cottonwood and live pine habitat compared to burned habitat. Predicted cumulative nest survival rates and nest success are consistent with previous studies on Lewis’s Woodpeckers in Montana  and British Columbia , but not consistent with studies on Lewis’s Woodpeckers in Colorado, South Dakota, and Idaho [27, 25] (Table 6). A study in Montana found that nest success and productivity of Lewis’s Woodpeckers within cottonwood habitat was high and comparable to our results from cottonwood habitat in British Columbia . We monitored Lewis’s Woodpeckers at some of the same sites in British Columbia as Zhu et al. , who combined data from live pine and burned habitat, and found that nest success and productivity was relatively low and comparable to the combined results from live pine and burned habitat from our study (Table 6). Breeding performance of Lewis’s Woodpeckers in British Columbia may have previously been underestimated in Zhu et al.’s  study because they monitored fewer nests in the more productive cottonwood habitat compared to this study. In contrast to our results, a study in Colorado  found that Lewis’s Woodpeckers within cottonwood habitat in Colorado had much lower nest success and productivity than those within cottonwood habitat in our study (Table 6). This could be due to higher grazing pressure within cottonwood habitat in Colorado, which has diminished the understory vegetation, likely reducing food availability for woodpecker nest provisioning . A study in South Dakota  found that nests within burned habitat (17–20 years post-fire) had high nest success and productivity, and a study in Idaho  found that Lewis’s Woodpeckers in burned habitat (2–7 years post-fire) had high nest success but moderate productivity. Both of these findings are in contrast to our results of low nest success and low productivity in burned habitat (10–15 years post-fire) in British Columbia (Table 6). It is not likely that differences across regions in nest success and productivity within burned sites can be explained by differences in fire history because this study and the study from South Dakota , which both took place within “older burns” (forests burned at a high intensity between 10–20 years prior) had opposite results. These regional differences in breeding performance mean that conservation planning for this species of concern may require recommendations that target regionally important factors affecting productivity.
Here we show that cottonwood habitat may provide critical habitat for Lewis’s Woodpeckers in British Columbia, because hatching success, daily nest survival, and productivity of Lewis’s Woodpeckers is high in this habitat type. In British Columbia, cottonwood habitat comprises less than 20% of the designated critical habitat, which includes all three habitat types . Therefore, management for high quality nesting habitat must consider both the protection of spatially limited strips of riparian vegetation along river valleys and protection of the more widespread, but potentially lower quality live pine and burned areas. Previous work has suggested that the Lewis’s Woodpecker is a burn-specialist species. However, burned forests do not appear to provide the most productive habitat in this region. Although non-native European Starlings are suspected to play a role in the decline of Lewis’s Woodpecker populations, our data suggest they do not influence breeding performance differences among habitats in British Columbia. Our results instead suggest that the low productivity particularly in burned habitat may be due, in part, to limitation of nesting cavities. We suggest that other factors such as the quality of food provisioned to nestlings or variation in nest exposure to predators should be examined. We highlight that our results differ from those in some other regions, suggesting that conclusions regarding critical habitat for this species at risk should use region-specific breeding performance data.