Glenn Tattersall at Brock University in Canada and his team of colleagues started photographing captive toucans with infrared cameras, which show warm areas as bright and cool areas as dark.
When temperatures rise, the scientists noticed that the toucans' bills glowed with radiated heat as warm blood flowed into them. When temperatures dropped, the blood flow would stop and the bills would go dark.
Paleontologists have hypothesized that some dinosaurs used bony head ornaments to shed heat in the same way, but due to a lack of live subjects they haven't been able to prove this theory.
Images courtesy Glenn Tattersall, via National Geographic