Evidence That Free-Ranging Common Nighthawks May Enter Torpor

Abstract

Low ambient temperatures or food shortages are known to induce some normally endothermic mammals and birds (principally hummingbirds and goatsuckers) to enter torpor, a strategy that allows animals to reduce metabolism and hence body temperature as a means of saving energy. The goatsuckers (Caprimulgidae) are of special interest in this respect because several species are known to use torpor (Reinertsen 1983) and one, the common poorwill (Phalaenoptilus nuttallii), can allegedly hibernate (Jaeger 1948; 1949), the only avian species thought to do so. Recent studies of free-ranging poorwills indicate that torpor is used regularly outside the breeding season (Brigham 1992; Csada and Brigham 1994) and on occasion by incubating or brooding birds (Kissner and Brigham 1993). In laboratory studies, Lasiewski and Dawson (1964), induced captive common nighthawks (Chordeiles minor) to enter torpor by starving them. However, only one of four birds that entered torpor survived a drop in body temperature below 25° C. Firman et al. (1993) found no evidence for the use of torpor by free-ranging nighthawks during a summer with exceptionally cold wet weather, when torpor is most likely to occur. While acknowledging that a lack of evidence does not prove these birds cannot use torpor, Firman et al. (1993) concluded that it was highly unlikely they did so under natural conditions. Here we report two observations of free-ranging nighthawks that appear to have entered torpor. During an early morning (between 0600 and 0700 PST) bird survey in the last half of June, 1985, KHM encountered what he first thought was a dead nighthawk at about 1000 m asl, approximately 16 km southwest of Merritt, British Columbia (120°55' W, 50°01' N). The site had been selectively logged, producing a relatively open canopy. The air temperature was approximately 4° C. An upright head position indicated that the bird (a female) was alive. The bird did not move when touched and although the nighthawk was not picked up, its dorsal surface was cold to the touch. On a return visit to the site several hours later, the bird was not present. In August 1986 at about 1600 MST, PCJ found a nighthawk on the ground within 3 m of the edge of a small unnamed lake about 2 km from Bragg Creek, Alberta (114°34' W, 50°55' N; 1300 m asl). The day was unseasonably cold with an ambient temperature of approximately 5° C and a strong wind from the north. The bird (sex unknown) was cold to the touch. It was definitely alive but made no attempt to escape when picked up. It was quickly returned to the roosting spot and was gone the next morning. Although both birds apparently moved following the initial observations, we cannot rule out the possibility that they were taken by predators as opposed to rewarming and flying away. There was no physical evidence (e.g., feathers) to suggest predation even though the plumage of this species is easily removed (R.M. Brigham unpub. data). Therefore we conclude that the likely intances of torpor observed were of a facultative nature (spontaneously reversible) and not pathological. We must therefore reconsider the conclusion that free-ranging nighthawks do not use torpor (Firman et al. 1993). Although the use of this energy-saving strategy by these birds is likely rare, as suggested from laboratory data by Lasiewski and Dawson (1964), we think it likely that at least under some conditions, nighthawks can allow their body temperatures to fall below normal levels. Apparently they can rewarm either passively in sunshine or by active metabolism to normal body temperature. As a result of this, Hickey's (1993) conclusion that whippoor-wills (Caprimulgus vociferus) do not use torpor, may also have to be revisited.