It's difficult to look at a vulture without thinking of some crotchety old man who is having a bad day.
For many years vultures have been characterized this way, not only because of their hunched appearance but also because of the conspicuous baldness. Those big, bare patches of skin make it difficult to be glamorous.
The exposed skin on a vulture's head – and, if you look closely, on its legs and chest too – does, however, serve a purpose. First of all, a vulture is not entirely bald: light down covers the skin that seems at a glance to be featherless.
For a long time ornithologists assumed the relative absence of feathers around vultures' heads made their dietary habits less problematic – if you dig around in bloody carcasses for lunch, a feathery head will be much harder to keep clean.
New research has suggested another purpose for bald patches: thermoregulation. Infrared photography of the griffon vulture has shown its bald patches to be much warmer than the rest of the bird. This is what you would expect: not insulated by thick feathers, they can serve to radiate away heat as needed.
When the bird gets particularly hot it can take flight and lose heat faster by virtue of the cold air rushing by that exposed skin. When it needs to keep the heat in, the vulture can reduce its exposed areas by drawing itself in tightly, with the head pulled back and the warm feathers circling around.
Black vultures and turkey vultures have another method of cooling off, but it's even less glamorous than being bald and hunched over: on hot days they urinate onto their legs.