Behavioural ecology and thermal physiology of Australian Owlet-Nightjars ('Aegotheles cristatus')

Abstract

The Australian owlet-nightjar ('Aegothelidae cristatus') is a sedentary, nocturnal, avian insectivore that uses a variety of adaptations to balance its energy budget. This has enabled this species to successfully inhabit diverse habitats throughout Australia. Owlet-nightjars are Caprimulgiformes, a group that typically have low metabolic rates and are capable of entering torpor to conserve energy. However, unlike many species of Caprimulgiformes, owlet-nightjars do not migrate and therefore must cope metabolically and behaviourally with seasonal variations in ambient temperature (Ta) and food resources. They are also unique amongst the Caprimulgiformes in that they are obligate cavity users year-round. I studied the behavioural and physiological qualities that enable this species to subsist in two dissimilar habitats, the semi-arid desert of central Australia and the comparatively cold, mesic, eucalypt woodlands atop the Northern Tablelands of NSW. I used radiotelemetry to locate diurnal roost sites, to track birds to determine home range, and to quantify body temperature (Tb) fluctuations in relation to roost and ambient thermal conditions. I compared characteristics of cavity roosts with randomly selected unoccupied cavities and used an information theoretic approach to assess variables which may be important for roost selection. Faecal samples collected from birds and cavity roosts at both locations were used to compare the diet and inferred foraging tactics between the two habitats in relation to arthropod availability. Finally, I measured the metabolic rate and thermal conductance of individual birds in the laboratory during both summer and winter using open-flow respirometry in the laboratory.