Abstract:
As air temperatures (Tair) increase, birds may face increasingly severe trade-offs between heat dissipation and other important behaviours (i.e. foraging and provisioning offspring), the cumulative fitness costs of which may ultimately drive population decline. Within habitats, the availability of cool microsites, and water may buffer individuals from the effects of high environmental temperatures (Tenv) under current and future climates. Costs associated with harsh environmental conditions and high Tair may also be buffered in group-living species, as load-lightening in social groups may allow individuals more time for their own self-maintenance. I investigated the hypothesis that home range features (shade and water) and social factors (group size) buffer physiological costs associated with high Tair. I monitored a population of White-browed Sparrow-weavers, Plocepasser mahali, (n=64 individuals from 15 colonies) in the Kalahari Desert of northern South Africa over an austral summer. The likelihood of shade-seeking and panting increased with increasing Tair, while the likelihood of foraging declined. Foraging intensity (peck rate per unit time foraging) decreased with the onset of panting; thus, overall peck rates per focal observation decreased as Tair increased. Shadier home ranges buffered the onset of panting, allowing individuals to forage less while maintaining overall peck rates. Birds with access to water foraged more (both in terms of likelihood of foraging and overall peck rate) across all Tair, and sought shade at hotter Tair, than birds without; but did not pant more. Individuals in larger groups were more likely to be observed foraging, panted more overall and sought shade more at low Tair and less at high Tair than those in smaller groups, but maintained similar overall peck rates. Peck rates per focal therefore declined with increasing Tair, were higher in birds with access to water, but were not affected by habitat or social factors. Taken together, these results suggest that birds in sunnier home ranges or larger groups may be forced to forage more at high Tair to maintain similar overall peck rates to those in shadier home ranges or in smaller groups, exposing them to higher thermal risk. Shade availability in home ranges may therefore buffer some impacts of increasing Tair under climate change, but impacts of water availability and social factors are less clear. Understanding how impacts of hot weather on behaviour are modified by habitat and social factors can inform models of range changes and population persistence under ongoing climate warming.
