Daily torpor in free-ranging whip-poor-wills (Caprimulgus vociferus).

The use of heterothermy is well documented in the order Caprimulgiformes, but there is conflicting information regarding whether whip-poor-wills are heterothermic. Consequently, we sought to rigorously examine the thermoregulatory abilities of this species. Our study was conducted in southeast South Dakota (42 degrees 47'N, 97 degrees 0'W), where 35 individuals were captured and outfitted with external, temperature-sensitive radio transmitters. We found evidence that whip-poor-wills used daily torpor during the autumn of 2000 and the spring of 2001 (n=12 torpor bouts, based on 346 bird-nights of observation). The average minimum skin temperature of two torpid whip-poor-wills (n=5 torpor bouts) in spring 2001 was 20.1 degrees +/-2.6 degrees C, and bouts of reduced skin temperature lasted an average of 360.0+/-93.7 min. The distribution of heterothermy within the Caprimulgiform phylogeny suggests that the trait is ancestral in the order. Specific heterothermic parameters, however, differ among the different species. In particular, the frequency of torpor use in whip-poor-wills is lower than for other species. These data suggest that several factors, including weather conditions and gender-specific reproductive ecology, influence the propensity of whip-poor-wills and other Caprimulgiformes to enter torpor.