Bill size variation in northern cardinals associated with anthropogenic drivers across North America.

Allen's rule predicts that homeotherms inhabiting cooler climates will have smaller appendages, while those inhabiting warmer climates will have larger appendages relative to body size. Birds' bills tend to be larger at lower latitudes, but few studies have tested whether modern climate change and urbanization affect bill size. Our study explored whether bill size in a wide-ranging bird would be larger in warmer, drier regions and increase with rising temperatures. Furthermore, we predicted that bill size would be larger in densely populated areas, due to urban heat island effects and the higher concentration of supplementary foods. Using measurements from 605 museum specimens, we explored the effects of climate and housing density on northern cardinal bill size over an 85-year period across the Linnaean subspecies' range. We quantified the geographic relationships between bill surface area, housing density, and minimum temperature using linear mixed effect models and geographically weighted regression. We then tested whether bill surface area changed due to housing density and temperature in three subregions (Chicago, IL., Washington, D.C., and Ithaca, NY). Across North America, cardinals occupying drier regions had larger bills, a pattern strongest in males. This relationship was mediated by temperature such that birds in warm, dry areas had larger bills than those in cool, dry areas. Over time, female cardinals' bill size increased with warming temperatures in Washington, D.C., and Ithaca. Bill size was smaller in developed areas of Chicago, but larger in Washington, D.C., while there was no pattern in Ithaca, NY. We found that climate and urbanization were strongly associated with bill size for a wide-ranging bird. These biogeographic relationships were characterized by sex-specific differences, varying relationships with housing density, and geographic variability. It is likely that anthropogenic pressures will continue to influence species, potentially promoting microevolutionary changes over space and time.