Comparative Patterns of Nestling Growth in White-Crowned Sparrows

Publisher Website
Google Scholar
Abstract
To examine the hypothesis that the growth rates of emberizines are physiologically maximized and hence geographically invariant, the eggs and nestlings of white-crowned sparrows (Z. leucophrys) from subarctic, subalpine and low-altitude montane populations were weighed. These data were augmented with published information on another subalpine population and a Pacific maritime population. Clutch sizes ranged from a mode of 3 in the south to 5 in the north. Geographic variation in fresh egg weight was closely correlated with the size of the adult female. Other sources of variation, if any, were small. The mean incubation period was geographically invariant (.apprx. 12.2 days). The weight (water) loss of eggs during incubation averaged 18.4%, without statistically significant differences among 4 localities. There were no consistent differences in the synchrony of hatching in clutches of 3, 4 or 5 eggs. Runt nestlings resulting from late hatching were atypical in the mainly 5-egg clutches in the subarctic but were more common in the unusual 5-egg clutches of subalpine populations. The postnatal growth curves of nestlings in all 6 sample populations were virtually congruent and thus independent of mean brood size and locality. It is unlikely that this would occur in such diverse populations unless growth rate were physiologically maximized. Greater diversity was expected if growth rates were submaximal and optimized in relation to the local environment.