Regularly drinking desert birds have greater evaporative cooling capacity and higher heat tolerance limits than non-drinking species

1. Surface water is a critical resource for many birds inhabiting arid regions, but the implications of regular drinking and dependence on surface water for the evolution of thermal physiology remain largely unexplored.

2. We hypothesized that avian thermoregulation in the heat has evolved in tandem with the use of surface water and predicted that (a) regularly drinking species have a greater capacity to elevate rates of evaporative water loss (EWL) compared to non-drinking species, and (b) heat tolerance limits (HTLs) are higher among drinking species. To test these predictions, we quantified thermoregulatory responses to high air temperature (T_a) in 12 species of passerines from the South African arid zone and combined these with values for an additional five species. We categorized each species as either: (a) water-dependent, regularly drinking, or (b) waterindependent, occasional-/non-drinking.

3. In support of our predictions, both conventional and phylogenetically independent analyses revealed that evaporative scope (the ratio of maximum EWL to minimum thermoneutral EWL) during acute heat exposure was significantly higher among drinking species. Moreover, evaporative scope was significantly and positively related to HTL (i.e. maximum air temperature tolerated before the onset of severe hyperthermia).

4. These findings offer new insights into the co-evolution of water dependence, movement ecology and thermal physiology in the context of trade-offs between dehydration avoidance and resistance to lethal hyperthermia in hot desert habitats.