Our study showed that many wintering and year-round resident species used residential areas and adjacent urban forest fragments. We found that more species preferred residential habitats to forest fragments in the winter. Out of the 41 total species we observed, 13 significantly preferred residential habitats and only 6 significantly preferred the adjacent urban forest fragments. Typical year-round resident species such as the Northern Cardinal, Chipping Sparrow, and Eastern Bluebird, unsurprisingly, showed preference for residential areas. These are common urban-affiliated bird species (Seewagen et al. 2010). Chipping sparrows and Northern Cardinals are granivores and may have preferred residential areas due to increases in supplementary feeding (i.e. bird feeders) and sparce vegetation in the residential sites. As seen in previous studies (Chace & Walsh 2006), granivores are common in residential habitat and thrive in human-influenced areas. A more surprising result was that a few understory and canopy insectivores, like the Yellow-rumped Warbler, significantly preferred the residential habitat over the forest fragments or did not prefer one habitat over the other, like the Pine Warbler and Ruby-crowned Kinglet. These migrant species were observed foraging in the mid-canopy, sparce understory vegetation, and on the ground in the residential areas.
The amount of tree canopy and other vegetation at our residential sites may have increased the abundance and diversity of resources available to birds, affecting their habitat preferences. Although such habitats typically include more impervious surface and mowed lawns than nearby forested fragments (Threlfall et al. 2012; Threlfall et al. 2016), and that native plants are often replaced by exotics (Threlfall et al. 2016), the residential areas in our study may have enough vegetation to support a variety of species. Residential areas that retain some native vegetation and vegetation structure in yards have been shown to provide habitat for many species (Belaire et al. 2014; Archer et al. 2019). Blair (1996) found that in the oak woodlands of California, total avian richness peaked under moderately disturbed conditions (e.g., golf courses and low-density, detached, single-unit homes).
Even though in our study the forest fragments contained more understory vegetation, ground habitat and slightly more canopy cover than residential sites, there may be enough vegetation in residential areas for some species in the winter. MacArthur (1958) showed that Yellow-rumped Warblers had more flexible foraging strategies than other eastern forest warblers. In wintering grounds, Yellow-rumped Warbler (called Myrtle Warblers in the MacArthur paper) foraged from beaches to forests and frequenting open ground. They were common under bayberry plants (Myrica pensylvanica) eating insects or feeding on berries (MacArthur 1958). Yellow-rumped Warblers in our study significantly preferred the residential areas to the urban forest fragments, displaying behavioral flexibility first recorded by MacArthur (1958). Although more canopy cover is associated with greater arthropod species richness and abundance (Turrini & Knop 2015), the 60 percent canopy cover in the residential neighborhoods may be enough habitat to support species that have flexible foraging strategies. Yellow-rumped Warblers may be foraging not only on arthropods, but some berries and even seeds in feeders. A previous study on the coast of California did show that urban winter bird communities commonly included insectivores like the Yellow-rumped Warbler and Ruby-crowned Kinglet (Kalinowski & Johnson 2010). These insectivores, in the California study, were associated positively with tall vegetation and negatively with road and building structure cover in the urban landscapes. Another unsurprising result was that the Eastern Bluebird preferred the residential areas. This species has been previously reported preferring open habitats, such as those found in residential neighborhoods (Gowaty & Plissner 2015). Therefore, residential yards have the potential to provide avian habitat for some insectivores during the winter, especially when canopy cover and vegetation structure is present.
However, other migrants and residents in the insectivore category, Hermit Thrushes, Ovenbirds, Northern Flickers, and Blue-headed Vireos, significantly preferred forest fragments. This suggests that urban forest fragments are still important to retain and manage in order to provide winter habitat for some species. Main et al. (2011) found that the bird community, including year-around species and migrants, in South Florida was negatively associated with native hammock forested fragments, but some species did prefer or showed no difference between hammocks and residential areas. In the South Florida study, birds of conservation concern, such as the migrating Prairie Warbler (Setophaga discolor) and the year-round Mangrove Cuckoo (Coccyzus minor), both preferred the native hammock forest fragments (Main, et al. 2011). In our study, Northern Flickers, the only ground foraging woodpecker, significantly preferred forest fragments. Studies have shown that Northern flickers are rare in Norther California urban and residential yards (Merenlender et al. 2009), so urban forest fragments are a valuable refuge for these species during the winter. Additionally, we only observed Ovenbirds, another common ground foraging species, in forest fragments. Previous work (Hostetler et al. 2005) has shown that increasing urban structures impacts the abundance of Ovenbirds and resulted in their extirpation from a previously wooded area in Gainesville, Florida when it was developed into an apartment complex. In their study, Hostetler et al. (2005) reported Ovenbird occurrences declined from 32 to 3 sightings after an apartment complex was built. Although treed neighborhoods in our study were able to support a variety of wintering species in Florida, forest fragments may be vital for some species, like the Ovenbird and Northern Flicker.
With the foraging guilds, 28 percent of omnivores and 57 percent of granivores preferred residential habitat. In winter, supplementary feeding from humans has been reported to meet the energy need of many granivore species (Lancaster & Rees 1979). Granivores, like the Northern Cardinal and Chipping Sparrow are able to forage on supplementary feeding from humans (i.e., bird feeders) (Jokimäki & Suhonen 1998). This is one of the most obvious reasons for the observation that omnivorous and granivorous bird species generally prefer urban habitats and why these species preferred the residential neighborhoods to the urban forest fragments in our study. Galbraith et al. (2015) showed that supplementary feeding, from things like bird feeders, restructured urban bird communities. Our results were consistent with other studies in that most granivore species significantly preferred residential habitat and many omnivore species either significantly preferred residential habitat or showed trends in the residential direction.
All 4 frugivore species, Baltimore Orioles, Cedar Waxwings, Northern Mockingbirds, and American Robins, showed a significant preference for residential habitat during the winter. Residential yards that retain some native plant species, but also have exotic plants can provide alternative foraging opportunities for birds, especially frugivores. A study by Andreu et al. (2017) showed that residential areas in Gainesville contained 70 species of trees, while forest areas only contained 42 species. Although the forest areas had more native species, the residential areas contained more species, especially exotics (Andreu et al., 2017). In another urban study, residential habitats in Australia were found to contain higher plant species richness because of the ornamental shrubs and exotic plants (Threlfall et al. 2016). Gleditsch and Carlo (2011) also found a positive association between the abundance of some birds and an exotic plant species, honeysuckles (Lonicera spp.), in Pennsylvania. This was not only beneficial for the honeysuckle species, but in areas of high honeysuckle density, native trees had 30% higher fruit removal rates compared to areas with low density of honeysuckle (Gleditsch & Carlo 2011). While the exotic fruits might attract the birds, native fruit bearing species also benefitted. Exotic plant species may have provided extra foraging opportunities for frugivores in Gainesville residential areas.
We also found that some winter migrants that were observed in another migration study (Buron, unpublished data) preferred similar habitats during both seasons. Species like the Ovenbird and Eastern Towhee had the same habitat preference during migration and winter. The Eastern Towhee almost exclusively used forest fragments during the winter and migration. This results corresponds with other studies that claim that Eastern Towhees are threatened by urban development (Seewagen et al. 2010), suggesting forest fragments are important stopover and wintering habitat for this species. Further, the Black-and-white Warbler significantly preferred forest fragments during migration (Buron, unpublished data), but in this study, during the winter period, they had no significant preference. Also, the Common Yellowthroat significantly preferred fragments during winter, but in the previous migration study, it had no preference. Further investigation is needed to understand this difference in habitat preference during migration and winter.
Of note, we did not address whether a species would be foraging in a residential area if there were not nearby forest fragments. All our residential surveys were near urban forest fragments; it is possible that a heavily treed residential area that is situated far from forest fragments may not contain certain species during the winter. More comparative studies are needed to ascertain whether larger landscape features affect bird distributions.
4.1. Recommendations for Urban Planning.
This study found that a variety of winter birds can use both residential areas with tree canopy and forest fragments in the urban matrix. Even migrating species that are classified as interior forest breeders, like the Black-and-white Warbler (Archer et al. 2019), still used residential trees and urban forest fragments as overwintering habitat in Gainesville. Other under-canopy insectivore species like the Palm Warbler and Yellow-rumped Warbler significantly preferred residential habitat. Another common under-canopy insectivore, the Ruby-crowned Kinglet, used both forest fragments and residential habitats frequently. These results point to the importance of not just looking at patterns in groups or guilds of species, but also studying individual species response. Studies documenting individual species preference can point to results that are missed from just looking at bird community and guild studies. Overall, this study adds to the evidence that vegetation at residential yards and conserving forested habitats in the urban matrix, impact bird diversity and contribute to conservation of vital bird habitat. Although planners and developers often prioritize large forest fragments for conservation, small forest fragments and landscaping in residential yards can help provide winter habitat for migrant and resident bird species.