The costs of keeping cool: behavioural trade-offs between foraging and thermoregulation are associated with significant mass losses in an arid-zone bird
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Avian responses to high environmental temperatures include retreating to cooler microsites and/or increasing rates of evaporative heat dissipation via panting, both of which may affect foraging success. We hypothesized that behavioural trade-offs constrain the maintenance of avian body condition in hot environments, and tested predictions arising from this hypothesis for male Southern Yellow-billed Hornbills (Tockus leucomelas) breeding in the Kalahari Desert. Operative temperatures experienced by the hornbills varied by up to 13 °C among four microsite categories used by foraging males. Lower prey capture rates while panting and reductions associated with the occupancy of off-ground microsites, resulted in sharp declines in foraging efficiency during hot weather. Consequently, male body mass (Mb) gain between sunrise and sunset decreased with increasing daily maximum air temperature (Tmax), from ~ 5% when Tmax < 25 °C to zero when Tmax = 38.4 °C. Overnight Mb loss averaged ~ 4.5% irrespective of Tmax, creating a situation where nett 24-h Mb loss approached 5% on extremely hot days. These findings support the notion that temperature is a major determinant of body condition for arid-zone birds. Moreover, the strong temperature dependence of foraging success and body condition among male hornbills provisioning nests raises the possibility that male behavioural trade-offs translate into equally strong effects of hot weather on female condition and nest success. Our results also reveal how rapid anthropogenic climate change is likely to substantially decrease the probability of arid-zone birds like hornbills being able to successfully provision nests while maintaining their own condition.
KeywordsTockus leucomelas Southern yellow-billed hornbill Climate change Microsite Body condition
We thank the De Bruin family of Leeupan farm, the Kotze family of Rus en Vrede farm and the Kalahari Research Trust for support and access provided to their land. We thank R. O. Martin for discussions of operative temperature measurement and S-J. Schultz, T. Vink, D. Portelli and J. Bruning for their help with data collection in the field. This research was conducted under ethical clearance from the Animal Ethics Committee, University of Cape Town, permit no. 2012/V44/PH and 2013/V24/PR. The study was carried out on private land (Kuruman River Reserve, Leeupan Guest Farm and Rus en Vrede farm) with permission of the landowners and of the Northern Cape Department of Environment and Nature Conservation of South Africa (permit numbers 995/2012, 660/2013 and 1166/2013). This research was primarily funded by the DST-NRF Centre of Excellence at the FitzPatrick Institute. Any opinions, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Research Foundation.
Author contribution statement
SJC, AEM and TMFNVDV conceived and designed the study. TMFNVDV collected data in the field. TMFNVDV analysed the data with input from SJC. SJC, TMFNVDV, and AEM together wrote the manuscript.
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