Drinking dependency shapes behavioural thermoregulatory trade-offs in four arid-zone larks

Desert birds obtain water either solely from diet and metabolic water (drinking independent), or by drinking scarce surface water (drinking dependent). However, the implications of drinking dependency for behavioural trade-offs made by birds in the context of thermoregulation is not fully understood. We hypothesized that drinking dependency would influence foraging and thermoregulatory behaviours in birds with otherwise similar ecologies, because species that drink may be able to use evaporative cooling more readily than non-drinking species. We predicted that drinking birds would a) utilize respiratory evaporative cooling (panting) earlier than non-drinking species and b) continue to forage in the sun at hotter temperatures than non-drinking species. We tested these predictions using scan samples and focal observations of four species of lark (Alaudidae; two drinking species and two non-drinking species) across a range of air temperatures (Tair, 16.2-43.2 degrees C) in the arid Tankwa Karoo National Park, South Africa. Our results indicate that drinking species forage in the sun at hotter air temperatures and begin panting at cooler air temperatures than non-drinking species. Foraging intensity was higher in the sun than the shade for all species, so this potentially allows drinking species to maintain higher food intake rates during hot weather. Drinking species may therefore be less vulnerable to behavioural thermoregulatory trade-offs and more resilient to increased temperatures under climate change. However, drinking species rely on free surface water and as a result could be more vulnerable than non-drinking species if water resources diminish as climate change progresses.