Taking the heat: how do parent birds mitigate costs of breeding at high temperatures?

Murphy, Benjamin

Taking the heat: how do parent birds mitigate costs of breeding at high temperatures?

Thesis / Dissertation

2026

Publisher

University of Cape Town

Department

Department of Biological Sciences

Faculty

Faculty of Science

Abstract

Rising air temperatures (Tair) associated with climate change are negatively affecting animals' fitness globally. Animals dwelling in arid zone environments are particularly at risk, as these environments are often characterised by extremely hot and dry conditions. In these regions, resident animals face Tair that regularly approaches or exceeds their preferred body temperatures (Tb), leading to significantly increased thermoregulatory costs. Behavioural thermoregulation during hot weather may also trade off against other important activities such as foraging and offspring provisioning. In birds, these trade-offs are commonly associated with negative impacts of high temperature on nestling growth and decreased reproductive success. However, despite the thermal constraints of a hot environment which result in reduced foraging and provisioning, fork-tailed drongos (Dicrurus adsimilis) in the Kalahari Desert, southern Africa, appear able to buffer nestling growth against declines during hot weather. An under-explored aspect of species' resilience to climate change is the ability of individuals to flexibly change behaviour in response to adverse conditions. Drongos present an exceptional opportunity to investigate the scope of behavioural flexibility in buffering negative impacts on reproductive success, measured as nestling growth, in an extremely challenging arid environment. In this thesis I use a combination of experiments, focal observations, and nestling and adult morphometrics, which I collected over six consecutive summer field seasons (2018-2023), to investigate how parent drongos succeed in buffering nestling growth against high temperatures. My study population in the Kalahari is habituated to human presence and colour-ringed, allowing experimental protocols to be conducted with precise examination of individual behaviours. I investigate whether parents mitigate negative effects of high Tair for nestlings by a) changing how they forage and provision, in relation to time-of-day, thereby offsetting reductions in provisioning rate during hot weather; b) changing foraging microsite use and provisioning decisions in relation to flight-distance and load-sizes per provisioning trip, thereby reducing heat load to themselves and improving provisioning efficiency during hot weather; c) managing nest temperature by increasing nest attendance in the context of increasing Tair, thereby reducing offsprings' exposure to high heat loads. Finally, I investigate d) the impact of hydration status on foraging and provisioning decisions by drongos in relation to increasing Tair. I found that drongos flexibly adjust their behaviour to mitigate the effects of heat on hotter days. As the maximum Tair reached during the day increased, drongos increased provisioning during the Morning to compensate for declines in provisioning during hot Midday and Afternoon periods. Additionally, drongos increased the load size provisioned per trip to the nest as Tair increased during Afternoons, helping to maintain stable food delivery to offspring on hot days while reducing their thermal costs of travel. However, provisioning decisions were not influenced by flight distance, irrespective of Tair. Although, drongos retreated to shady locations in response to increasing Tair, their foraging efficiency remained the same in the shade versus in the sun. Nest attendance also increased as Tair increased during Midday-afternoon periods, which allowed parent drongos to buffer nest temperatures and minimize nestlings' exposure to thermal extremes that could impair their growth on very hot days. Water, rather than energy, might be a key limiting resource shaping flexibility of birds' behaviour in this study system, as drongos that were fed water-enriched mealworms (which improved their hydration status) reduced foraging activity at high temperatures, in contrast to birds fed non-enriched mealworms (lower hydration). The behavioural flexibility I documented in this study appears to currently provide drongos with resilience against rising temperatures, in terms of the quality (measured as size and mass) of the offspring they produce. The work therefore provides further evidence that behavioural flexibility may increase species resilience to global change. However, it is unclear if the plasticity exhibited by drongos breeding in the Kalahari will remain sufficient to buffer negative effects of temperature on breeding outcomes as climate warming and extreme heat events intensify into the future. For example, as early morning temperatures rise with increasing daily temperature maxima, the available window for compensatory provisioning may shrink, limiting parents' capacity to meet nestlings' needs throughout the day. Additionally, climate-driven declines in prey availability, the increasing need to attend nestlings during hot weather and limited opportunities to replenish water reserves could compromise adult condition, likely limiting scope of further flexibility. Overall, these limits to behavioural flexibility may impair animals' ability to adapt to ongoing climate pressures, highlighting the need for further research on the costs, benefits, and limitations of such adjustments for the long-term viability of species in a changing climate.

Keywords

climate change

drongos

Reference:

Collections

PhD / Doctoral

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