The selective pressures acting on phenotypes are complex and can vary both spatially and temporally. To elucidate relationships among environmental conditions and selection on a complex morphological trait, we explored spatial and temporal variation in avian bill structure in a common generalist songbird, the Dark-eyed Junco (Junco hyemalis). We measured bill length, width, and depth, and calculated bill surface area for >800 museum specimens collected in California from 1905 to 1980. We then determined which environmental variables (precipitation, temperature, habitat type) acting over which temporal scales (seasonal, annual, hemi-decadal, decadal) explained variation in these measures of bill morphology. Although we had predicted that relationships between environmental parameters and selection on the bill structure would reflect either foraging ecology or thermoregulatory needs, the patterns that we found were more complex and varied with season and among the different bill traits examined. Temperature was consistently a more important predictor of bill morphology than precipitation, although overall support for temperature was still weak. While bill surface area was related to habitat type, linear measures of bill morphology were related to temperature maxima, minima, and variability. Bill morphology was related to temperature maxima in summer; in contrast, both temperature maxima and minima were supported in winter models. Of the climate variables identified as important in our analyses, support was strongest for the measure of decadal temperature variability. The strong relationship between vegetative community and bill surface area revealed by our analyses as well as the unexpected role of decadal temperature variability indicate that consideration of the large-scale context—ecology and climate—in which complex phenotypic traits occur may reveal important patterns of selection that are not evident from studies of more isolated components of natural systems.
- Junco hyemalis
- Temporal scale