Shared temporal increases in bill size among songbirds of the San Francisco bay area are due to different seasonal selective pressures

Museum specimens offer a unique and powerful tool for understanding the impact of anthropogenic change on populations over time. Morphological traits can be impacted by many different environmental variables that are difficult to separate from one another as potential driving factors. Comparative analyses among similar species jointly experiencing change in the same environmental variables can help pinpoint the selective pressures driving temporal morphological change. We assessed temporal change in bill size, tarsus length, and body size between six species of songbirds from the San Francisco Bay Area over the past 150 years. Wing length, as a proxy for body size, exhibited idiosyncratic temporal changes among species. In contrast, we found a significant increase in bill surface area across all but one species. Quantile regression analyses on bill size variation additionally revealed that temporal increases over the past century have been driven by increases in the largest bill sizes in some species, but increases in the smallest bills over time in others. The climate variables best explaining temporal change in bill size also differed among species with some species responding more to changing summer variables (e.g. maximum annual temperature) and others in response to a changing winter climate. These results together suggest that different sympatric, resident bird species may be experiencing temporal morphological change in response to selective pressures experienced at different seasons. Our finding provides support for the season of critical thermal stress hypothesis that suggests variation in functional traits will be shaped by the season that imposes the greatest selective force on a population. Overall, this study has important implications for future research on the role of bills in thermoregulation and for conservation efforts based on the adaptive capacity of birds to respond to climate change.

Methods
The dataset includes morphological measurements made from 1,001 museum specimens at Museum of Vertebrate Zoology, University of California Berkeley and California Academy of Sciences. Measurements were all made using digital calipers. Climate variables for each specimen come from the PRISM climate database and were extracted using locality info from each specimen.