Daily Torpor in a Free-ranging Goatsucker, the Common Poorwill (Phalaenoptilus nuttallii)

Abstract

Numerous laboratory studies show that common poorwills (Caprimulgidae: Phalaenoptilus nuttallii) are capable of entering daily torpor when deprived of food. Using temperature-sensitive radio transmitters, I measured the skin temperature of free-ranging birds under natural conditions to test three hypotheses about the use of torpor by poorwills. I predicted that (1) poorwills would enter torpor only in "energy emergencies" (defined as birds with low body mass), (2) only the nonincubating or brooding member of a pair would use torpor during the breeding season, and (3) poorwills would be less likely to enter torpor on moonlit nights when longer periods of activity can be sustained. My results show that adult poorwills of both sexes enter torpor regularly in April, May, and September, but not during the breeding season. I found no evidence that torpor was used only in energy emergencies or that the lunar cycle influenced the use of torpor. Skin temperatures regularly dropped below 10°C and in one instance fell below 3 C. On one occasion an individual bird remained torpid for at least 36 h. I found limited evidence suggesting that the temperature at twilight, but not insect abundance, can be used to predict whether birds will remain active or enter torpor.