Migratory Birds on Wintering Grounds in Tropics Have a Higher Basal Metabolic Rate than Tropical Residents

AbstractLong-distance endurance flight in migratory birds requires numerous physiological adaptations. Maintaining the necessary physiological machinery may require increased energy expenditure, which could manifest as an elevated basal metabolic rate (BMR). Comparing the BMR of long-distance migrants and residents using global BMR datasets is complicated by the fact that data for migrants typically come from higher latitudes and colder regions than those for nonmigrants. Separating the effects of ambient temperature (T_a) and migratory tendency on BMR is challenging because of the well-documented high phenotypic plasticity of avian BMR, which is reflected in the negative relationship between BMR and T_a. We hypothesized that tropical migrants would have a higher BMR than residents because of the cost of maintaining a more enduring and/or flexible physiology. Additionally, according to the climatic variability hypothesis (CVH), BMR plasticity should be greater in regions with more variable climates. To test these hypotheses, we measured BMR in 130 sedentary species and 25 migratory species from two remote areas in Vietnam that differ in climate. As expected, we found that in both sites, migrants on their wintering grounds maintained a higher mass-independent BMR compared with tropical residents. Moreover, the BMR of wintering migrants was not lower than their BMR on their breeding grounds. Sedentary species inhabiting a milder and more stable climate had lower BMR than those in colder and more variable environments. The individual long-term repeatability of BMR in the milder climate was higher than that in the area with a more variable climate, which is consistent with the CVH.