Nocturnal reductions in body temperature in high-elevation Neotropical birds

Seasonal and daily fluctuations in environmental temperature can affect the fitness of endotherms by increasing metabolic costs and energetic requirements. Consequently, some species adopt strategies that function to minimize costs, including minor circadian fluctuations in body temperature (Tb) and facultative reductions in Tb, known as heterothermy. The geographic and taxonomic patterns of variation in Tb are poorly-known, especially in the Neotropics. We investigated the diurnal variation in Tb of small birds inhabiting high-elevation Neotropical montane forests which must cope with predictably cool nighttime temperatures. Two-thirds of the individuals we measured lowered their Tb at night, and changes were greater when differences between daytime and nighttime ambient temperatures were greater. Our study expands the taxonomic and geographic scope of documented thermoregulatory flexibility in birds by demonstrating that even in the Neotropics, some montane birds may routinely adopt energy-saving physiological strategies. Such data are important to understanding and interpreting biogeographic patterns and behavior of tropical birds.