Ecological correlates of torpor use among five caprimulgiform birds

We review recent studies of torpor use by free-ranging caprimulgiform birds in North America and Australia: common poorwill (45 g), whip-poor-will (55 g), common nighthawk (80 g), Australian owlet-nightjar (50 g), and Tawny Frogmouth (500 g). Reproductive activity, ambient temperature, body size, and prey abundance influence the energy status of endotherms and may be correlated with torpor use under natural conditions; our review suggests that no one factor is most important. To date, most studies have been correlational in the wild, and experimental studies at multiple locales are now needed to resolve uncertainties concerning the evolutionary and ecological significance of torpor.