Seasonal use of torpor by free-ranging Australian owlet-nightjars (Aegotheles cristatus).

Abstract

With the exception of some data for common poorwills (Phalaenoptilus nuttallii) and anecdotal reports for a few other species, knowledge about the use of torpor by free-ranging birds is limited. Our study was designed to assess the use of torpor by free-ranging Australian owlet-nightjars (Aegotheles cristatus). We selected this species for study because of their relatively small body size (50 g), arthropod diet, nocturnal sedentary nature, taxonomic affiliation with other birds for whom the use of torpor is well documented, use of cavity roosts, and because of the cold winter (mean July minimum ambient temperature [T(a)] of approximately 0 degrees C) in the study area. We tracked 12 owlet-nightjars carrying temperature-sensitive transmitters for a total of 906 bird-days (range of 15-115 d per individual). Five different individuals entered torpor on 96 d in total. Torpor bouts occurred only between May 8 and September 8, the coldest period of the year. The lowest skin temperature (T(skin)) recorded for any bird was 19.6 degrees C, and the lowest core temperature was 22.4 degrees C. Surprisingly, torpor was rarely used at night because birds usually foraged then. Instead, torpor typically began near dawn, even on cold nights. Torpor bouts beginning at dawn lasted approximately 4 h. On 36% of days when torpor was used at dawn, birds reentered torpor later in the day. Torpor was not used during the breeding season, but this period also corresponds to the warm part of the year. There were no distinct daily minimum, maximum, or mean T(a) thresholds that could be used to reliably distinguish days when dawn torpor was used from those when it was not, although torpor was commonly employed when daily minimum T(a) fell below 3.9 degrees C. Our results show that even though Australia is typically thought of as a warm continent, at least some of the avifauna use torpor as a regular means of saving energy. We hypothesise that the reasons for this species' use of torpor include its ability to remain active all night foraging, either for terrestrial arthropods while walking or for flying insects taken on the wing, and/or its habit of roosting in cavities, which allows them to remain hidden in the daytime.