Seasonal Patterns of Time and Energy Allocation by Birds

Publisher Website
Google Scholar
Abstract
We evaluated two competing hypotheses concerning how field metabolic rate (FMR) of birds should vary seasonally. The reallocation hypothesis predicts little seasonal variation in FMR, whereas the increased demand hypothesis predicts that FMR should reach an annual maximum during the breeding season. In both yellow-eyed juncos (Junco phaeonotus) and dark-eyed juncos (Junco hyemalis), winter FMR averaged 70.5 kJ/d, which is not significantly different from each species' average breeding season value and thus confirms the reallocation hypothesis for these species. The avian literature suggests that diet determines which of the two hypotheses applies to species that breed in seasonal environments. Birds that forage on prey that are difficult to capture follow the increased demand hypothesis, whereas other species exhibit reallocation. To maintain energy balance, yellow-eyed juncos had to forage longer during the winter than during the breeding season (6. O vs. 4.5 h/d) because their rate of metabolizable energy gain was 22% lower when foraging on seeds in winter than when foraging on insects during the breeding season (11.8 vs. 15.1 kJ per hour spent foraging, respectively). We used behavioral, meteorological, and laboratory metabolism data to calculate the energy devoted to thermoregulation under field conditions during the winter. Our analysis revealed that all of the heat produced as a by-product of physical activity contributed to thermoregulation. Basal metabolism and thermoregulatory costs combined constituted the juncos' entire winter FMR, with thermoregulatory costs averaging 57% of FMR in yellow-eyed juncos and 49% in dark-eyed juncos. Thus, behavior has no net energy cost at low temperatures in juncos. Our data also suggest that winter is a period of greater energy stringency for juncos than is the breeding season.