Data from: 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.