Free-ranging common nighthawks use torpor

Abstract

Bouts of torpor are characterized by substantial reductions in body temperature and metabolic rate. Many birds in the order Caprimulgiformes use torpor; however, there is conflicting evidence whether or not Common Nighthawks (Chordeiles minor) have this ability. The purpose of our study was to examine whether free-ranging nighthawks enter torpor. Three out of eight birds that were monitored entered torpor on a total of four occasions. Our results suggest that torpor is not used as commonly as it is in other species of Caprimulgids providing incentive for further research to explain the ecological determinants of torpor use in Caprimulgids.

Introduction

Endothermy is associated with high energetic costs during cold weather, particularly in small mammals and birds with large surface area: volume ratios (Song et al., 1995). Some birds allow their body temperature (T_b) to fall below normothermic levels during facultative hypothermic responses; in a hypothermic state their metabolic rate decreases and substantial energetic savings accrue (Prinzinger et al., 1991). Birds may use ambient heat to arouse from facultative hypothermia (Geiser and Drury, 2003) but they also have the ability to spontaneously increase T_b using metabolically generated heat (Hudson, 1978). Facultative hypothermia is often induced by energetically stressful conditions including cold ambient temperatures (T_a) and/or reduced food availability (Brigham et al., 2000).

Two types of daily facultative hypothermic responses have been described for birds. Torpor is defined as a state of reduced responsiveness to external stimuli associated with body temperatures below normothermia (Geiser and Ruf, 1995; McKechnie and Lovegrove, 2002). Rest-phase hypothermia is a second category that is characterized by normal responsiveness to external stimuli and shallower reductions in T_b below normothermia (McKechnie and Lovegrove, 2002). Distinguishing between rest-phase hypothermia and torpor in free-ranging animals is usually impossible because it is difficult to obtain behavioural observations (but see Pierce and Vogt, 1993). Therefore, we evaluated the ability of birds to facultatively reduce their T_b and use the term torpor to define this state.

The use of torpor is common in the avian Order Caprimulgiformes. Torpor occurs in Common Poorwills (Phalaenoptilus nuttallii, Brigham, 1992), Whip-poor-wills (Caprimulgus vociferus, Lane et al., in press), European Nightjars (Caprimulgus europaeus, Peiponen and Bosley, 1964), Australian Owlet-nightjars (Aegotheles cristatus, Brigham et al., 2000), and Tawny Frogmouths (Podargus strigoides, Körtner et al (2000), Körtner et al (2001)). The Common Nighthawk (Chordeiles minor) is a member of the Caprimulgiformes for which there is conflicting evidence about torpor use. Free-ranging nighthawks in the Okanagan Valley of British Columbia did not use torpor (mean minimum T_skin=34.5±0.5°C [SE], 39 bird-nights) even though local environmental conditions were unusually harsh (above average precipitation) during the study period (Firman et al., 1993). In the laboratory, three of four nighthawks died when forced into torpor by food deprivation with a 28–34% drop in body mass (Lasiewski and Dawson, 1964). Even though one nighthawk entered and aroused from torpor in this study (its T_b fell to 18°C at a T_a of 16°C), the authors suggested that torpor would not be employed in the wild because only one individual survived. Conversely, an energetic model that attempted to predict the foraging time of nighthawks suggested that these birds may use torpor to balance their energy budget (Aldridge and Brigham, 1991). In addition, Brigham et al. (1995) made anecdotal observations of two nighthawks that were cold to the touch; however, these were not confirmed bouts of torpor because the fate of these birds was undetermined.

In light of the conflicting evidence, our objective was to determine if Common Nighthawks enter torpor under relatively harsh natural conditions. We predicted that nighthawks would enter bouts of torpor on cold mornings and nights because their crepuscular foraging bouts would be less successful, and their thermoregulatory costs would be higher, relative to warm mornings and evenings (Körtner and Geiser, 2000).