Free-ranging common nighthawks use torpor
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 (Tb) 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 Tb using metabolically generated heat (Hudson, 1978). Facultative hypothermia is often induced by energetically stressful conditions including cold ambient temperatures (Ta) 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 Tb 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 Tb 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 Tskin=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 Tb fell to 18°C at a Ta 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).
Section snippets
Study area
This study was conducted in the West Block of Cypress Hills Provincial Park, Saskatchewan (49°34′N and 109°53′W) after the nesting season during the summers of 2000 and 2001. The local vegetation of the Cypress Hills consists of rolling hills of short grass fescue prairie, intermixed with stands of lodgepole pine (Pinus contorta), white spruce (Picea glauca), trembling aspen (Populus tremuloides) and balsam popular (Populus balsmifera). This area is also characterized by colder temperatures and
Results
The mean mass of the fifteen male and four female nighthawks captured was 82.7±1.5 g. Tskin data from seven males and one female were collected during 28 bird-mornings and 31 bird-nights (range of 4–13 bird-mornings and nights per individual). The average Tact was 33.6±0.6°C. Similar to Brigham et al. (2000), Tskin appears to underestimate Tb by about 3°C since the average Tact was ∼3°C below the normothermic temperature recorded under laboratory conditions (Tb=34–40°C over Tas ranging from 2°C
Discussion
Our results show that free-ranging nighthawks have the ability to enter torpor. Studying torpor under laboratory conditions allows researchers to quantify metabolic attributes of torpor bouts, but studies of free-ranging animals, like the present one, are essential because some animals that enter torpor under natural conditions are reluctant to do so in the lab (Geiser et al., 2000).
Nighthawks in the Cypress Hills did not appear to use torpor in response to cool Tas. Our finding that only four
Acknowledgments
We are grateful to C.M. Voss, S. Martinez, J. Adams, R.G. Poulin and D.J.H. Sleep, for field assistance and to members of the “bird and bat lab” for their support, encouragement and guidance. The work was funded by a Natural Sciences and Engineering Research Council (NSERC) operating grant to RMB, and an NSERC Undergraduate Student Research Award to QEF. We thank F. Geiser, A.E. McKechnie, J.E. Lane and K.A. Edwards for helpful comments that greatly improved the article.
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