Non-evaporative heat dissipation across the beaks and casques of large forest hornbills

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 (T_air) 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 (T_beak) tracked T_air below 24–25 °C, but at higher T_air, the T_beak – T_air 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 T_air above which the T_beak – T_air gradient increases (T_inflection) by combining our data with published and three unpublished values. We found that T_inflection decreases with increasing body mass (M_b), with the relationship best described by the linear regression model T_inflection = −9.134log₁₀M_b + 50.83, with M_b in g.