Winter and Breeding-Season Energetics of Nonmigratory White-Crowned Sparrows

Abstract

The lives of most temperate-latitude birds are organized on an annual cycle that includes reproduction, molt, dispersal and/or migration, and winter maintenance. How food and energy requirements vary with stage of the annual cycle, and which stage (if any) constitutes an energy bottleneck, are poorly known, but two alternative hypotheses have generally been recognized (Masman et al. 1989). The reallocation holds that abundant food and moderate temperatures during the breeding season result in decreased thermoregulatory and foraging costs, which allows energy to be reallocated to breeding. This hypothesis predicts little seasonal variation in field metabolic rate (FMR). The increased-demand holds that breeding results in a substantial increase in the energy demand of adults. By breeding during the season with maximum food availability, parent birds are able to meet their increased energy demands. This hypothesis predicts that FMR should reach a seasonal maximum during the breeding season. Early attempts to deduce the annual cycle in FMR by extrapolating laboratory metabolic measurements to the field produced conflicting patterns. Kendeigh (1973) derived a monthly energy budget for central Illinois House Sparrows (Passer domesticus) in which FMR attained an annual maximum of 117 kJ day-' in January and February and then gradually decreased to an annual nadir of 83 kJ day-' in August. The seasonal change in FMR in Kendeigh's analysis was driven largely by seasonal variation in basal metabolic rate and thermostatic costs. In contrast, Farner (1980) derived an annual energy budget for a migratory race of Whitecrowned Sparrow (Zonotrichia leucophrys gambelii) in which FMR was 100 kJ day-' during the winter and breeding seasons. Subsequent studies that estimated FMR using more direct techniques revealed a diversity of patterns (see Weathers and Sullivan 1993), with some species following the House Sparrow model and others showing little seasonal change in FMR. Which pattern is followed appears to be related to diet in that FMR increases little during the breeding season in species with energetically economical foraging modes, whereas FMR of species that feed on mobile animal prey is higher during the breeding season than during winter (Weathers and