Our data confirm that hornbills, li

Our data confirm that hornbills, like other birds, can regulate heat exchange from their beaks,
using them as thermal radiators when air temperatures are high, but restricting heat loss during
cold ambient conditions. The large beaks of both hornbills and toucans are highly vascularised,
and control of blood is regulated by vasoconstriction and vasodilation processes [8]. Bird beakscontain branches of major cranial nerves (e.g. the trigeminal nerve) and associated sensory
structures [22,23], which require a supply of oxygenated blood. Vascularity of the beak is therefore
almost certainly a plesiomorphic avian trait [24]. Heat exchange from the beak occurs in
all species investigated to date [1–5,25]. The ability to regulate heat loss via the beak is probably
most essential in large-beaked birds because of the need to conserve heat at low temperatures
[1,26]. These hornbill beaks therefore function as ‘thermal windows’ [1], similar to betterknown
examples such as elephant ears [27], bat wings [28], lemur indices [29], dolphin fins
[30,31], hummingbird eyes and axial region [32] and bird legs [33–35]. Hughes [11] suggested
that the large beaks of toucans and hornbills evolved the ability to facilitate thermoregulation
as an exaptation on top of foraging function [11]. However, given the thermal constraints associated
with heat loss from a very large beak in the cold, it is also possible that beak size and control
over blood flow into the beak evolved in tandem under selective pressure to prevent heat
loss during cold periods.
Although both hornbills and toucans have the ability to regulate heat exchange through
their large beaks, the threshold temperature for vascular recruitment and relative beak size differed
markedly between the two species. Toco Toucans were able to dissipate on average 60%
of total heat loss via the beak [4]. The heat dissipated via the beak in Southern Yellow-billed
Hornbills was much less: on average ~ 8% of total heat loss (maximum 19.9% at Ta of 33.0°C in
one individual). We calculated maximum rate per unit surface area under comparable convective
conditions using data presented in the figures in the paper by Tattersall et al. [4] and found
that toucans dissipated as much as 61.4Wm-2 via their beaks, whereas the Yellow-billed Hornbills
we studied only dissipated a maximum of 25.1Wm-2 via the beak. We suggest three possible
explanations for these differences in heat exchange capacity in hornbills and toucans. First,
toucans allowed beak surface temperature to rise at considerably cooler air temperature than
hornbills (20–25°C compared to 30.5–31°C), allowing them to achieve a steeper gradient