Prey availability affects daily torpor by free-ranging Australian owlet-nightjars (Aegotheles cristatus)

Food availability, ambient temperatures (Ta), and prevailing weather conditions have long been presumed to influence torpor use. To a large extent, this is based on measurements in the laboratory of animals placed on restricted diets and kept at low Ta. Information on the determinants of torpor employment in the field is limited. We assessed winter torpor by insectivorous, free-ranging Australian owlet-nightjars (Aegotheles cristatus; 22 birds, 834 bird-days over six winters). Birds in three habitats were investigated to test whether torpor use is affected by annual Ta, rainfall, and arthropod abundance. Owletnightjars entered daily torpor regularly at all sites. Torpor frequency, depth and bout duration were greatest during two periods with lower arthropod abundance, providing rare evidence of the link between food availability and torpor patterns of wild birds. Temporal organization of torpor was similar among sites, and nocturnal torpor was more frequent than previously reported. Our findings quantitatively demonstrate that reduced food resources affect torpor usage independently from Ta, and support the view that food availability is a primary ecological determinant of torpor use in the wild.