Avian thermoregulation in the heat: metabolism, evaporative cooling and gular flutter in two small owls

The thermoregulatory responses of owls to heat stress have been the subject of few studies. Although noctumality buffers desert-dwelling owls from significant heat stress during activity, roost sites in tree and cactus cavities or in deep shade provide only limited refuge from high environmental temperatures during the day. We measured thermoregulatory responses to acute heat stress in two species of small owls, the elf owl (Micrathene whiffler) and the western screechowl (Megascops kennicottii), which occupy the Sonoran Desert of southwestern North America, an area of extreme heat and aridity. We exposed wild-caught birds to progressively increasing air temperatures (T-a) and measured resting metabolic rate (RMR), evaporative water loss (EWL), body temperature (T-b) and heat tolerance limits (HTL; the maximum T-a reached). Comparatively low RMR values were observed in both species, T-b approximated T-a at 40 degrees C and mild hyperthermia occurred as T-a was increased toward the HTL. Elf owls and screechowls reached HTLs of 48 and 52 degrees C, respectively, and RMR increased to 1.5 and 1.9 times thermoneutral values. Rates of EWL at the HTL allowed for the dissipation of 167-198% of metabolic heat production (MHP). Gular flutter was used as the primary means of evaporative heat dissipation and produced large increases in evaporative heat loss (44-100%), accompanied by only small increases (<5%) in RMR. These small, cavity-nesting owls have thermoregulatory capacities that are intermediate between those of the open-ground nesting nightjars and the passerines that occupy the same ecosystem.