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Roost type influences torpor use by Australian owlet-nightjars

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Abstract

Australian owlet-nightjars (Aegotheles cristatus; ∼50 g) are one of only a few avian species that roost in cavities year-round and regularly enter torpor. Cavity roosts act as thermal buffers, and roost type likely affects energy expenditure of small birds. We used radiotelemetry to locate diurnal winter roost sites of owlet-nightjars in central Australia and to measure body (T b) and skin (T skin) temperature. We also recorded ambient temperature inside (T IN) and outside roosts. Individual owlet-nightjars used one to seven different roosts (tracking time 3–10 weeks), selecting either rock crevices (four birds) or tree hollows (four birds), or switching between the two roost types (seven birds). Rock crevices (T IN +9°C to +33°C) were warmer and thermally more stable than tree hollows (T IN −4.0°C to +37°C). Torpor, often expressed by a reduction of T skin/T b by >10°C for 3–4 h at dawn, was influenced by roost selection; torpor use in tree hollows was almost twice that in rock crevices. Despite the potential energy savings accrued from roosting in well-insulated cavities, owlet-nightjars roosted in tree hollows more often (65% bird days, n = 398) than in rock crevices (35% bird days, n = 211). Lower costs of arousal from torpor via passive rewarming and basking and decreased risk of predation are two possible explanations for the preference to roost in tree hollows. We provide the first evidence for the influence of cavity roost selection on torpor use in a free-ranging bird and show that roost selection and thermal biology are strongly interrelated in determining energy expenditure.

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Acknowledgements

Thanks to Fred Harvey for extensive field assistance. This study was facilitated by funding from the University of New England, the National Science and Engineering Council of Canada and the Australian Research Council. Small grants were provided by Birds Australia, the Royal Zoological Society of New South Wales and the Australian Bird Study Association. Permits for this research were issued by the University of New England Animal Ethics Committee, the Northern Territory Parks and Wildlife Service and the Australia Bird and Bat Banding Scheme. Accommodation at Ormiston was provided by the NT Parks and Wildlife Service.

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Author notes

  1. Chris R. Pavey

    Present address: CSIRO, PO Box 2111, Alice Springs, NT, 0871, Australia

Authors and Affiliations

  1. Centre for Behavioural and Physiological Ecology, Zoology, University of New England, Armidale, NSW, 2351, Australia

    Lisa I. Doucette & Fritz Geiser

  2. Department of Biology, University of Regina, Regina, SK, S4S 0A2, Canada

    R. Mark Brigham

  3. Biodiversity Conservation, NRETAS, PO Box 1120, Alice Springs, NT, 0871, Australia

    Chris R. Pavey

Authors
  1. Lisa I. Doucette
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  2. R. Mark Brigham
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  3. Chris R. Pavey
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  4. Fritz Geiser
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Correspondence to Lisa I. Doucette.

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Communicated by: Sven Thatje

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Doucette, L.I., Brigham, R.M., Pavey, C.R. et al. Roost type influences torpor use by Australian owlet-nightjars. Naturwissenschaften 98, 845 (2011). https://doi.org/10.1007/s00114-011-0835-7

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