The beak and unfeathered skin as heat radiators in the Southern Ground-hornbill

The avian beak is increasingly recognised as an important organ for thermoregulation, particularly in disproportionately large-beaked taxa such as toucans and hornbills. We used infrared thermography to test the prediction that Southern Ground-hornbills (Bucorvus leadbeateri) physiologically regulate the surface temperature of their beak (Tbeak), as well as that of their facial (Tfacial) and gular skin (Tgular) in such a way that these surfaces provide avenues for non-evaporative heat dissipation in warm weather. Our data, collected over air temperatures (Ta) ranging from 11 °C to 36 °C, supported these predictions. At Ta < 20 °C, Tbeak tracked Ta, but rapidly increased to values 6-12 °C above Ta at Ta = 20-22 °C. The Tbeak-Ta gradient was maintained at approximately 7 °C between Ta = 22 °C and 28 °C, before decreasing linearly with Ta > 28 °C. We also found evidence for active regulation of Tfacial and Tgular, with both surfaces regulated at approximately constant increments abov...