| Common Poorwills (Phalaenoptilus nuttallii) are nocturnal insectivorous birds that breed across western North America, typically in and or semiarid habitats characterized by open landscapes and limited overhead cover. Poorwills are unique amongst all birds because they are able to enter torpor for prolonged periods.; Lunar illumination strongly influenced poorwill behaviour; poorwills were more active and vocal when the moon was full than otherwise, and breeding also appeared to be synchronized with lunar phase historically. Nevertheless, lunar phase did not correlate with torpor use or inactivity. I found little evidence that poorwills at higher latitudes breed earlier than other caprimulgids where their breeding ranges overlap, suggesting that the ability to enter torpor does not facilitate earlier breeding at those latitudes, although it probably does in the southernmost portions of their range. I also found circumstantial evidence that some poorwills may overwinter in warm locales north of the species presumed winter range.; The use of torpor was commonplace, as it was used by one or more birds on 60% of all nights through the calendar year, and over a period that included all but July, August, and September. During torpor bouts body temperature dropped rapidly to near ambient temperature for ambient temperatures above ca. 6°C. Ambient temperature was a highly significant predictor of both torpor use and the density of flying insects available to poorwills at night, and birds used torpor at times when foraging became increasingly inefficient owing to a reduction in prey abundance.; Inactivity tended to be seasonal in nature, but one or more poorwills remained entirely inactive on 36% of all nights through the calendar year. The period during which birds were sometimes inactive lasted 6 months, and birds often remained continuously inactive for one to two weeks at a time during winter. Poorwills selected winter roosts that were open to the south or southwest. Consequently, because most days were sunny, inactive birds were usually passively warmed by the sun, so that body temperature typically rose to the point of euthermia during the afternoon, then dropped at sunset as the inactive bird re-entered torpor. When birds were experimentally shaded at their winter roosts to prevent them from arousing passively, they remained continuously torpid, with torpor bouts interrupted by brief periods of euthermia at 4 to 7 day intervals. Estimates of energy consumption suggest that when winters are warm, overwintering in sunny locations maximizes energetic savings in comparison to roosting in shade and arousing periodically, in part because birds are euthermic at times of the day when torpor is the least beneficial energetically, owing to high ambient and hence body temperature. Moreover, passive daily arousal facilitates activity at sunset when conditions warrant, and is probably an adaptation to surface dormancy.; Several birds were killed by predators during my study, but torpid ones were not more likely to be depredated than were active ones, and concealment owing to cryptic coloration and lack of movement apparently confers a greater degree of protection to inactive birds than may have been anticipated. My findings demonstrate that poorwills can, and sometimes do, remain entirely inactive for long periods in winter, and that during those times they exhibit physiological patterns typical of mammalian hibernation. (Abstract shortened by UMI.)... |
