ABSTRACT Aim Urban environments provide unique challenges and opportunities for wildlife, including altered thermal regimes caused by the urban heat island (UHI) effect. Gloger's rule predicts that animals in warmer environments will exhibit more pheomelanin‐based (reddish‐brown) plumage. For species with colour polymorphisms, local warming may shift morph frequencies towards those with more pheomelanin. Eastern screech‐owls ( Megascops asio ) have rufous, brown, and grey morphs, with a strong latitudinal cline where grey morphs increase in the northern areas of the species range, likely due to lower metabolic demands. We predicted that the proportion of rufous morphs will increase in warmer UHIs compared to the cooler surrounding rural areas. Location Eastern North America. Taxon Eastern screech‐owl ( Megascops asio ). Methods We used community science images from Cornell's Macaulay Library and iNaturalist from throughout M. asio asio 's range to quantify proportions of rufous versus non‐rufous morphs across 26 UHIs and their adjacent rural area. Results Populations of eastern screech‐owls within UHIs had significantly more rufous individuals compared to the surrounding rural areas, and there was a strong latitudinal cline in morph frequency. However, site‐specific climatic variables did not predict differences in morphs between UHI and their surrounding rural sites. Main Conclusions Warmer urban microclimates within UHIs may favour pheomelanic morphs with higher metabolic demands, consistent with Gloger's rule. These results suggest that even relatively small‐scale environmental differences can shape selection pressures among populations. These findings underscore the potential for urban environments to influence phenotypic variation and offer insight into how climate‐driven selection may operate at fine spatial scales.
Briggs et al. (Wed,) studied this question.