Energetics and Water Balance in Free-Living Tropical Hummingbirds

In Costa Rica's humid, Caribbean lowlands, we used the doubly-labeled water (DLW) technique to measure field metabolic rate (FMR) and water influx of free-living Crowned Woodnymphs (Thalurania colombica) (mean mass = 4.90 g) and Bronze-tailed Plumeleteers (Chalybura urochrysia) (mean mass = 7.23 g). FMR averaged 37.9 kJ/day in woodnymphs (n = 9) and was 57.9 kJ/day in a single plumeleteer. The former value is 26% higher than expected from the birds' mass, based on other DLW studies. Water influx averaged 2,392 ml/(kg·day) in woodnymphs (n = 12) and 2,001 ml/(kg·day) in plumeleteers (n = 2). These are the highest water flux rates measured for any bird and are equivalent to turning over, respectively, 366% and 304% of the birds' total body water content each day.