Resource scarcity increases foraging activity despite thermal risk in an arid-adapted bird

Behavioural flexibility is considered key for species to cope with the effects of climate change. Periods of resource scarcity, such as during climate change-driven droughts, may force animals to increase their foraging activity to meet energetic demands. However, doing so may increase the thermal risk inherent to the trade-off between foraging and exposure to higher temperatures. We use biologging data (internal body temperature and activity budgets classified via accelerometers) from a population of wild vulturine guineafowl (Acryllium vulturinum) to quantify this trade-off across varying resource availability (quantified through NDVI) over 2.5 years, including a period of severe drought. We start by linking internal body temperature measurements to external operative temperature to identify a thermal threshold--where body temperature reaches a maximum--at an external temperature of 33.8{degrees}C. We interpret this threshold as the point where birds must prioritise heat dissipation (physiologically and/or behaviourally) to avoid hyperthermia. Relevantly, we find that a larger part of the day exceeds this external thermal threshold as resources decline. We then show that guineafowl feeding activity positively correlates with resource scarcity, suggesting the birds compensate for diminishing resources by increasing foraging time investment, despite thermal risks. However, the compensatory foraging does not appear to buffer the birds against food shortfalls, as birds still lose weight as resource scarcity increases. Sublethal effects (e.g., body mass loss) of compounding environmental stressors have fitness consequences (e.g., postponement of breeding, as was noted in our population). Our results could point to an evolutionary mismatch; the projected intensification of resource scarcity and higher temperatures under climate change may alter trade-offs in such a way that existing physiological and behavioural responses to cyclical dry seasons are no longer adaptive, in vulturine guineafowl, and in other arid-system organisms more broadly.