Torpor in Australian birds

Abstract

Energy-conserving torpor is characterized by pronounced reductions in body temperature and metabolic rate and, in Australian birds, is known to occur in the Caprimulgiformes (spotted nightjar, Australian owlet-nightjar, tawny frogmouth), Apodiformes (white-throated needletail) and the Passeriformes (dusky woodswallow). Anecdotal evidence suggests that it also may occur in the white-fronted honeyeater, crimson chat, banded whiteface, red-capped robin, white-backed swallow, mistletoebird, and perhaps welcome swallow. Daily torpor (bouts lasting for several hours) appears to be the most common pattern, although anecdotal evidence indicates that white-backed swallows can undergo prolonged torpor. Diurnal birds enter torpor only during the night but nocturnal birds may use it by the day and/or night, and often in more than one bout/day. Body temperatures fall from ~38–41°C during activity to ~29°C during torpor in spotted nightjars, tawny frogmouths, dusky woodswallows and white-throated needletails, and to ~22°C in Australian owlet-nightjars. In the spotted nightjar, a reduction in Tb by ~10°C resulted in a 75% reduction in metabolic rate, emphasizing energy conservation potential. Since torpor is likely to be more crucial for the survival of small birds, a detailed understanding of its use is important, not only for physiologists but also ecologists and wildlife managers. It is thus disappointing that so much information on torpidity in Australian birds is anecdotal, and that so little effort has been made to characterize its patterns and quantify the resulting energy savings and survival benefits for birds in the wild.