Feather macrostructure is marginally correlated with temperature range but not urbanization across California.

Urban environments are often associated with resource and environmental differences providing potential novel selection pressures compared to adjacent unmodified landscapes. While these characteristics (e.g., heat islands, reduced vegetation) can contribute to differences in certain behaviors, morphology, or physiological traits, there is mixed evidence on how and to what extent populations are responding. In this study, we compared the feather morphology of Dark-eyed Junco (Junco hyemalis) populations established across an urbanization gradient. We examined whether differential temperature regimes, related to urbanization, correspond with significant variations to the proportion of down. We sampled ventral and dorsal feathers from 256 individuals throughout central and southern California at varying degrees of urbanization. Dorsal feathers had a higher proportion of down compared to ventral feathers, but did not differ between populations. Urbanization did not significantly correlate with feather morphology. Ventral feathers had a greater proportion of down as the range of temperature increased, but this correlation was marginal. Our results show that despite urbanization altering fine-scale habitat conditions, these did not correspond with rapid feather morphological variations. Whether this is the case for other feather types or across species is still unknown but would provide insight into the complex effects of urbanization on wildlife biology.