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 ( T beak ), as well as that of their facial ( T facial ) and gular skin ( T gular ) in such a way that these surfaces provide avenues for non‐evaporative heat dissipation in warm weather. Our data, collected over air temperatures ( T a ) ranging from 11°C to 36°C, supported these predictions. At T a  < 20°C, T beak tracked T a , but rapidly increased to values 6–12°C above T a at T a  = 20–22°C. The T beak – T a gradient was maintained at approximately 7°C between T a  = 22°C and 28°C, before decreasing linearly with T a  > 28°C. We also found evidence for active regulation of T facial and T gular , with both surfaces regulated at approximately constant increments above T a up to inflection T a values of 22°C and 29°C, respectively. The presence of inflection T a values corresponding with abrupt changes in physiological regulation of these surface areas suggests a high degree of physiological control. Estimates of heat exchange suggest that SGH may be able to dissipate up to 75% of basal metabolic rate via these surfaces, confiming the beak and areas of unfeathered skin play an important thermoregulatory role in these large, distinctive and threatened birds.