Ubiquity in a bird species is often an excuse to overlook them, no matter how weird or wonderful they may initially appear. See them a few times and they don’t quite grab your attention anymore. The yellow-billed hornbill is one such bird, and the fact that over 50 of them may be encountered during any one game drive at Londolozi can lead to them being disregarded as a talking point. Yet there is far more to these front-heavy birds than meets the eye. Their peculiar hole-nesting behaviour is a regular conversation starter, but we’ll skip that today and focus on the most striking part of their anatomy; the bill.

A female yellow-billed hornbill, told apart from the male by her smaller beak. Juveniles also have relatively small beaks so can be difficult to sex, but the wear-and-tear on this individual ‘s upper mandible mark her as an adult. Photograph by James Tyrrell

It’s pretty clear where hornbills get their name from. Their heavy beaks are an immediate eye-catcher, and interestingly enough the hornbills are the only family in which the first and second neck vertebrae are fused; probably in order to provide more support for the bill.
As curious as the bill structure is, it’s what goes on inside it that is truly fascinating. It has been found that hornbills use their bills as one of their primary thermoregulatory mechanisms.
Birds don’t sweat like we do. Their main ways of cooling down are gular fluttering (the bird version of panting) and vasodilation (the opening of blood vessels close to the surface of their skin to dispel heat, particularly in the beak). Although most birds show some degree of vasodilation and constriction in their beaks, birds with unusually large beaks tend to make far better use of this technique.
Enter the hornbills.

A male yellow-billed hornbill (note the larger beak) presenting food at his nest site.

A recent study that looked at yellow-billed hornbills in the Kalahari desert wanted to see to what extent the birds used their beaks for heat loss. Using thermal imaging, scientists from the University of Cape Town monitored heat changes in 18 hornbills caught in the wild over a range of temperatures.

The top left image shows how the hornbill is losing almost no heat through its beak when temperatures are particularly low. Vasoconstriction has taken place to restrict blood flow to the bill. When the temperature starts climbing into the 30s however, the hornbill floods its beak with blood in order to dissipate body heat (bottom left). Photo Credit: Tanja van de Ven et al.

Another bird that uses its bill as a major thermoregulatory device is the Toco Toucan of South America, although the toucan’s beak represents a much higher proportion of its body surface area (30-50%), so by default it accounts for more heat loss than in yellow-billed hornbills, whose bills are only around 5-10% of their body surface area. A slightly hardened keratin layer in hornbills’ beaks would also slightly inhibit heat loss, but as they are generally seen to inhabit hardier environments than toucans, their bill necessarily needs to be slightly stronger, hence the reinforcement.

A Toco Toucan; almost the quintessential toucan one thinks of when one hears the name. Compare the relative size of the beak to the body and you can understand how these birds would lose proportionately more heat through their bills than their African counterparts the hornbills. Photograph by Charles J Sharp

Toucans generally inhabit more tropical climes than yellow-billed hornbills, and gular fluttering, which is a form of evaporative cooling, is probably less effective as a heat-loss mechanism in the species, since the humidity gradient will not be as steep between a moist membrane (throat) and the ambient atmosphere. They therefore need to rely far more heavily on the heat transfer mechanism of the bill, and up to 60% of their heat loss can be accounted for in this way!

Hornbills can be rather comical in appearance. Photograph by Alex Jordan

This is all quite a simplified discussion on the beak as a cooling mechanism in hornbills. The process is a bit more complicated than it sounds, and there are probably more factors involved than one would initially assume. Scientists conducting the study hypothesised that the dependance on the beak as a cooling mechanism may even vary greatly between populations of the same species.

We don’t carry thermal imaging cameras with us on the Londolozi Land Rovers, but if we had some it would be fascinating to take a few shots of the local hornbills through the seasons, on cold days and hot, and see what they are getting up to. Londolozi doesn’t have the temperature extremes as the Kalahari, but I’m pretty certain on the winter mornings, when we’re all bundled up in blankets and hot water bottles on game drive and are desperate for a mug of hot chocolate, the hornbill population is copying their cousins ‘ from further west, massively limiting that blood flow to the beak. And at the height of summer when we’re down to our bare essential clothing, those big yellow bills are full of blood, pumping heat out into the atmosphere as fast as they can!