Do Free-Ranging Common Nighthawks Enter Torpor?

There is conflicting evidence as to whether Common Nighthawks (Chordeiles minor) can enter torpor. The purpose of this study was to determine if torpor is used by free-ranging individuals under natural conditions. Nighthawks were monitored from June until August 1990 near Okanagan Falls, British Columbia, using temperature sensitive radiotransmitters. Record-high precipitation in 1990 apparently stressed the birds energetically by preventing foraging during poor weather and by reducing the abundance of the main prey item, caddisflies (Trichoptera). Energetic stress was manifested in several ways. Compared to previous years, nighthawks foraged diurnally, changed foraging habitats resulting in a broadening of the diet, and increased the duration of foraging periods. Furthermore, two tagged birds died, apparently of starvation. Despite indications that 1990 was a stressful year, the temperature of nighthawks never fell below homeothermic levels. If nighthawks are physiologically capable of entering torpor, an energetically stressful year would be expected to induce it. Our observations support the idea that they are not physiologically adapted to enter torpor as a means of energy conservation.