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 above Ta up to inflection Ta values of
22 °C and 29 °C, respectively. The presence of inflection Ta 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
unfeather skin play an important thermoregulatory role in these large,
distinctive and threatened birds.