Effects of food supplementation on behavioural decisions of hoopoe-larks in the Arabian Desert: balancing water, energy and thermoregulation

Patterns of time allocation to different activities can help reveal how natural selection has solved optimality problems that involve simultaneous environmental constraints. To investigate how time budgets of desert birds are affected by ambient temperature, lack of drinking water and low food availability, we provided food and water to hoopoe-larks, Alaemon alaudipes, in the Arabian Desert during years in which no larks reared young. We followed birds continuously from sunrise to sunset on unsupplemented and supplemented days, and recorded their behaviour every 15 s. Taking into account the variation in temperature between days, hoopoe-larks decreased foraging time by 13–29% of total daytime, and increased resting and preening time by 7–16 and 8%, respectively, when they had access to supplemental food. When birds had access to extra food, they began and ended their midday resting period when shade temperature was on average 2.2 C lower, and operative temperature was on average 3.1 C lower, than on unsupplemented days, a significant effect of food supplementation. We concluded that birds optimized time spent on foraging and thermoregulating based on a combination of physiological state variables, including body temperature, hydration state and level of energy reserves. Our results do not support a previous hypothesis that activity budgets of desert birds are dictated by thermal constraints alone.