Seasonality of metabolic capacity in resident tropical birds

Abstract

Temperature seasonality drives variation in metabolic activity of temperate zone resident bird species. Contrary to predictions from life history theory, energetic capacity is not maximal during the breeding season, but is instead highest during winter when survival is the animal's primary concern. Tropical residents do not experience the large annual temperature cycles characteristic of temperate zones. Reflecting their relatively constant environment, we predicted that tropical birds would either: 1) show no annual variation in metabolic capacity, or 2) show increased metabolic capacity during the breeding season. We collected data at the Smithsonian Tropical Research Institute station in Bocas del Toro, Republic of Panama (9°N) during either February (winter) or June (summer) on several avian species including several taxonomic groups and dietary preference. In all cases, birds measured in winter had lower metabolic capacity than the same species measured in summer. We conclude that although breeding efforts are less intense in tropical birds, that they are able to increase metabolic effort toward breeding over maintenance during the rest of the year.