Legs are primary heat exchangers in nest-bound White storks

Endotherms must manage heat fluxes under changing environmental conditions to maintain safe core body temperatures. An important mechanism for energy management is peripheral heterothermy. Despite growing evidence that birds' bare body parts such as legs and bills can serve as effective thermal windows (i.e. areas of controlled heat loss), the function and importance of such body regions likely differ across species and ecological contexts. This might be particularly important for birds breeding in exposed locations (such as open nests) that promote rapid heat gain and loss. Combining thermal imaging with weather data we assess the thermoregulatory role of various body parts (eye region, bill, gular skin, and legs) in White storks (Ciconia ciconia) breeding in Southwestern Iberia. In doing so, we also test whether adults and nestlings differ in their capacity to modulate surface temperature (T_surf) through these regions across a wide range of environmental temperatures (T_air = 15-38 °C). Our analyses showed that White storks finely tuned T_surf of all the regions tested in response to T_air, with nestlings showing an overall poorer capacity to modulate T_surf under colder conditions (T_air < 20 °C). Notably, only the legs served as effective thermal windows in adult storks, with dry heat loss through these appendages representing up to 70 % of basal metabolic rate. Thus, passive heat exchange through the legs can significantly contribute to alleviate heat stress in nest-bound White storks. Yet, further work is needed to better understand the ontogeny of thermal windows in the face of climate change.