UC Berkeley

The selective pressures acting on phenotypes are complex and can vary over space and time. To examine the effects of selection due to environmental conditions on avian bill morphology, we explored spatial and temporal variation in bill morphology in a common generalist songbird, the Dark-eyed Junco (Junco hyemalis). We measured bill length, width, and depth, and calculated surface area, for >800 museum specimens collected in the state of California from 1905-1980. We then examined which environmental variables (precipitation, temperature, and habitat type) at which temporal scales (seasonal, annual, hemi-decadal, and decadal) could explain variation in each measure of bill morphology. Although we predicted relationships consistent with selection on the bill for foraging utility and optimal thermoregulation, the patterns we found were more complex. The effects of the environmental factors examined varied with season and with the specific bill traits. Measures of habitat type were more strongly associated with bill morphology than were individual climate variables, and temperature was a more important predictor of bill morphology than precipitation. Bill surface area displayed stronger effects of environmental conditions than did linear measures of bill morphology. 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 in our models, the support for longer-term temperature variables, and the support for the importance of temperature variability suggests that in complex natural systems, large-scale context—ecology and climate—plays a strong role that is not seen by looking at its component parts alone.

Different environmental conditions produce different selective pressures. We explored patterns of life history variation along an elevation gradient to identify the factors contributing to this variation. We monitored breeding Dark-eyed Juncos in the Sierra Nevada mountains of California at sites from 1960 to 2660 m above sea level and compared breeding season length, temporal patterns of peak breeding activity, clutch size, brood size, nestling quality, and nest mortality among elevations. We also compared maximum and minimum daily temperature, daily snow depth, and monthly precipitation across the elevations to determine whether these abiotic factors could explain life history variation. We found small differences in breeding season length and in the pattern of reproductive timing among elevations. While breeding season at the intermediate elevation was intermediate in length, the pattern of peak breeding activity was not intermediate between the patterns observed at low and high elevations. The life history differences across elevations could not be explained solely by abiotic factors, but may be related to the effects of those factors on the birds' prey base or nesting sites, potentially exacerbated at lower elevations by the ongoing drought. We found no differences among elevations in clutch size, brood size, or nestling quality. Higher elevations had greater nest mortality, possibly due to severe weather. A computer simulation constructed to mimic the field system suggests that these mortality differences, in combination with the differences in breeding season length, contribute to substantial differences among elevations in reproductive success which may be difficult to observe in a field setting.

We then investigated whether variation in climatic conditions, breeding season length, and number of offspring produced per season in populations of junco breeding at different elevations led to variation in breeding synchrony, in extra-pair paternity, or in the use of a sexually-selected male signal, the amount of white in the tail. We also tested for genetic structure among the populations. Using 12 variable microsatellite loci, we found differences in extra-pair paternity rates among our populations, with a low extra-pair paternity rate (20% of nests) at high elevations, a high rate (57%) at middle elevations, and an intermediate rate (38%) at low elevations. We found no differences among elevations in mean values of tail white or tail white asymmetry, and no differences in a measure of the honesty of the signal (the strength of the correlation between tail white and either of two indices of male quality). Despite differences in the temporal patterns of breeding activity as well as in the length of the breeding season, we found no differences in breeding synchrony among elevations, and no association between the presence of extra-pair paternity and a brood's breeding synchrony. We detected no genetic differentiation among populations, indicating that consistent gene flow occurs between the populations. Persistent gene flow may explain the lack of differentiation in the tail white signal despite substantial differences in the potential strength of sexual selection among elevations.