Does Shape Matter? Breeding Habitat Use and Reproductive Performance of Gray Catbirds (dumetella Carolinensis) in Two Western Woodland Habitats

In the western United States, relatively few studies have comprehensively examined songbird performance in fragmented habitat, particularly within naturally fragmented systems. For this study, I used Gray Catbirds (Dumetella carolinensis) to distinguish differences in area use and reproductive performance in draw and floodplain habitat types, two spatially and physiognomically distinct deciduous woodlands. Naturally fragmented draw woodlands are bound by parallel hillsides, which creates a high edge-toarea ratio and a narrow, linear, configuration. By contrast, floodplain woodlands are wider and more contiguous. In this study, both draw and floodplain sites were surrounded by mixed-used grasslands and adjacent to each other, so they fell within the same landscape context and shared the same predominant, deciduous, species. However, these habitats differed according to configuration and floristics. To evaluate the potential effects of these differences, I monitored catbirds for three seasons (2014-2016) within both woodland habitat types. When compared to floodplain birds, results showed that draw birds had larger territories, exhibited lower daily nest survival rates, revealed delayed nest initiation patterns and had reduced fledgling success. I also tested local vegetation variables from surveys collected around the nest, to see if either configuration or local vegetation were potential mechanisms driving differential catbird productivity across habitat types. Tests of local vegetation showed that both down woody debris and mid-shrub canopy cover were significantly higher in draws than in the floodplains. However neither vegetation variable significantly influenced catbirds’ daily nest survival rates, and therefore, did not support local vegetation as the driving mechanism. Evidence instead pointed toward configuration. Configuration likely influenced territory size in the draws, as the vegetation is constrained due to the long, thin, shape of this habitat type. Draw nests were also subjected to more depredation events than floodplain nests. This could also be explained by territory size, since larger territories require birds to cover and defend more area, which could affect nest vigilance. Additionally, draws may be saturated with predators due to the limitation of available woodland habitat, and predators may easily enter the habitat from edge interfaces. Thus, these results corroborate some studies in the eastern U.S. which have also shown negative impacts of increased edge and fragmentation on songbird breeding performance. Overall, this study’s results can assist managers in understanding the repercussions of both natural and anthropogenically fragmented habitats, which will ultimately help manage breeding bird habitat in the future.