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Winter thermoregulation in free-ranging pygmy falcons in the Kalahari Desert

Abstract

Among birds, pronounced heterothermy tends to be used by small species belonging to phylogenetically older taxa and inhabiting environments with unpredictable and/or seasonally scarce food resources. Previous evidence suggests that a small arid-zone raptor, the pygmy falcon (Polihierax semitorquatus), enters torpor and decreases its body temperature (Tb) to below 31 °C on winter nights in the Kalahari Desert, where sub-zero night-time temperatures occur regularly. To confirm that this species has the capacity for heterothermy and to characterise its thermoregulatory patterns under natural conditions, we implanted temperature-sensitive radio transmitters into free-ranging falcons. Based on data from seven individuals, we found no evidence for the substantial reductions in Tb previously reported. The mean minimum rest-phase Tb during the study period was 37.89 ± 0.67 °C, we observed no instances of Tb < 36 °C, and air temperature did not significantly influence patterns of thermoregulation. Our results highlight intraspecific variability in heterothermy among birds and reveal that, contrary to previous suggestions, even in mid-winter pygmy falcons may not reduce Tb below normothermic levels.

Zusammenfassung

Thermoregulation im Winter beim freilebenden Halsband-Zwergfalken in der Kalahari Wüste

Bei Vögeln wird eine ausgeprägte Heterothermie tendenziell bei kleinen Arten vorgefunden, die zu den phylogenetisch älteren Taxa gehören und Habitate mit unvorhersehbaren und/oder saisonal knappen Nahrungsangeboten nutzen. Bisherige Hinweise lassen vermuten, dass eine kleine Greifvogelart arider Gebiete, der Halsband-Zwergfalke (Polihierax semitorquatus), während Winternächten in der Kalahari Wüste, wo nächtliche Minustemperaturen regelmäßig vorkommen, in einen Torporzustand verfällt und seine Körpertemperatur (Tb) auf unter 31 °C senkt. Um zu bestätigen, dass diese Art die Fähigkeit zur Heterothermie besitzt und um die thermoregulatorischen Muster unter natürlichen Bedingungen zu charakterisieren, implantierten wir temperatursensitive Radiotransmitter in freilebende Zwergfalken. Basierend auf den Daten von sieben Individuen konnten wir im Gegenteil zu vorherigen Untersuchungen keinen Hinweis für wesentliche Tb-Senkungen finden. Während unseres Untersuchungszeitraums lag das durchschnittliche Tb-Minimum in der Ruhephase bei 37,89 ± 0,67 °C Es gab keine Tb-Werte < 36 °C und die thermoregulatorischen Muster wurden nicht signifikant durch die Lufttemperatur beeinflusst. Unsere Ergebnisse heben die intraspezifische Variabilität der Heterothermie bei Vögeln hervor und zeigen entgegen vorherigen Vermutungen, dass Halsband-Zwergfalken auch inmitten des Winters ihre Tb nicht unterhalb ihres normothermen Bereichs absenken.

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Acknowledgements

We thank the Oppenheimer family, Tswalu Foundation and Tswalu Kalahari for permitting us to conduct this research on their property, and Mark Brigham and three anonymous reviewers for their constructive comments. This work was supported by funding from the DST-NRF Centre of Excellence at the FitzPatrick Institute to RLT and the National Research Foundation of South Africa (Grant Number 110506) to AEM. Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Research Foundation. All procedures in this study comply with South African law and were approved by the Animal Ethics Committee of the University of Cape Town (2018/v7/RT), Animal Ethics Committee of the University of Pretoria (EC011-18) and Research and Scientific Ethics Committee of the South African National Biodiversity Institute (P18/05), and FAUNA research permits from the Northern Cape Province’s Department of Environment and Nature Conservation.

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Correspondence to Andrew E. McKechnie.

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Deceased: Luke F. Arnot

Communicated by L. Fusani.

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Lund, J., Bolopo, D., Thomson, R.L. et al. Winter thermoregulation in free-ranging pygmy falcons in the Kalahari Desert. J Ornithol 161, 549–555 (2020). https://doi.org/10.1007/s10336-020-01755-y

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Keyword

  • Body temperature
  • Falconidae
  • Heterothermy
  • Telemetry
  • Torpor