Non-evaporative heat dissipation across the beaks and unfeathered skin of Old World vultures

Non-evaporative heat loss via the beak and unfeathered skin is an important avian thermoregulatory mechanism, but remains poorly studied in many taxa. We used infrared thermography to assess heat fluxes across the beak and unfeathered head and neck regions in three African vultures: Cape Vulture (Gyps coprotheres), Lappet-faced Vulture (Torgos tracheliotos) and White-backed Vulture (Gyps africanus). Heat loss was quantified over air temperatures (Tair) between 5°C and 36°C in vultures occupying large flight aviaries. In all three species, beak surface temperature (Tbeak) increased by up to >10°C above Tair, confirming vultures’ beaks serve as controllable heat radiators. For all three species, Tair emerged as a predictor of Tbeak, along with humidity (two species) and wind speed (one species). Maximum surface-area specific heat loss across the beak ranged from 98 W m−2 to 103 W m−2 and was equivalent to 4.7% (Cape), 5.9% (Lappet-faced) and 4.6% (White-backed) of allometrically predicted basal metabolic rate (BMR). For unfeathered skin, Tair, humidity and wind speed emerged as predictors for Cape Vultures, whereas evidence for effects of these environmental variables was more equivocal for Lappet-faced Vultures. Heat loss across head skin reached values of 148.7 W m−2 and 135 W m−2 for Cape and Lappet-faced Vultures, respectively, whereas corresponding rates for neck skin were 122.7 W m−2 and 118.4 W m−2. Maximum combined rates of heat loss across the beak and unfeathered skin were equivalent to 16.3% of allometrically-predicted BMR in Cape Vultures and 53.1% in Lappet-faced Vultures.