The Energetics of Australasian Swifts, Frogmouths, and Nightjars

Body temperature and rate of oxygen consumption were measured as a function of environmental temperature in two swiftlets from New Guinea, Collocalia esculenta and Collocalia vanikorensis, two nightjars from New Guinea, Eurostopodus mystacalis and Caprimulgus macrurus, and two frogmouths, Podargus strigoides from Australia and P. papuensis from New Guinea. Compared to rates expected from mass, basal rates of metabolism in these birds progressively decrease with an increase in mass. The reduction in basal rate may reflect food habits in swiftlets and food habits, sedentary habits, and small pectoral muscle masses in nightjars and frogmouths. The combination of small mass and low rate of metabolism might account for the variable body temperatures found in swiftlets, but no evidence of torpor has been found in these swiftlets, frogmouths, or nightjars. At cool temperatures, C. esculenta often clustered in the field and in the laboratory, a behavior that led to a 30% reduction in energy expenditure. Thermal conductances, which tend to be low in all species with low basal rates, often fell markedly in the largest frogmouth when exposed to ambient temperatures below 16° C.