Reproduction constrains the use of daily torpor by free-ranging common poorwills (Phalaenoptilus nuttallii) (Aves: Caprimulgidae)

The purpose of this study was to determine if common poorwills ( Phalaenoptilus nuttallii ) resist entering torpor during the breeding season. During the summers of 1991 and 1992, we studied poorwills in the Cypress Hills, Saskatchewan, Canada, near the northern limit of their distribution. Since poorwills are monogamous and share incubating and brooding responsibilities, we predicted that the non-incubating or non-brooding bird would enter torpor when stressed energetically (e.g. on cold and/or wet nights). Individuals carrying temperature-sensitive radio transmitters entered torpor significantly less often during the breeding season (two of 195 bird nights) than the non-breeding season (27 of 44 bird nights). During the breeding season we found no birds involved in an active nesting attempt in torpor. We conclude that reproduction constrains the use of torpor by adult birds, but why non-incubating and non-breeding birds did not enter remains unclear.