Abstract:
Heat loss across the beak is an important thermoregulatory mechanism among birds, particularly in large-beaked
taxa such as toucans (Ramphastidae) and hornbills (Bucerotidae). The number of species investigated remains
limited, as does our understanding of how the functional significance of this pathway varies with environmental
variables such as humidity, with little previous research on species inhabiting humid environments. We used
infrared thermography to test the hypothesis that large (600–1300 g) Afrotropical forest hornbills use their beaks
and casques as thermal radiators. We collected data over air temperatures (Tair) of 15–34 ◦C for wild-caught
trumpeter hornbills (Bycanistes bucinator) and captive-bred silvery-cheeked hornbills (Bycanistes brevis) and
black-casqued hornbills (Ceratogymna atrata). Surface temperatures of the beaks and casques (Tbeak) tracked Tair
below 24–25 ◦C, but at higher Tair, the Tbeak – Tair gradient increased to maximum values of 10–12 ◦C. Maximum
rates of beak heat loss were 2.5–3.8 W, equivalent to 31–83 % of estimated resting metabolic heat production.
Facial skin showed also evidence for active regulation of heat loss. We also analysed the scaling of the inflection
Tair above which the Tbeak – Tair gradient increases (Tinflection) by combining our data with published and three
unpublished values. We found that Tinflection decreases with increasing body mass (Mb), with the relationship best
described by the linear regression model Tinflection = 9.134log10Mb + 50.83, with Mb in g.
