Birds incubate eggs to provide a constant cozy environment for their developing offspring. The central heater for most eggs is the parent's broodpatch, an area of thick, bare, vascularized skin on the abdomen. When parents sit on their nest, the brood patch touches and warms the eggs. However, some species do not have a brood patch and resort to alternative heating systems to incubate their young. Most famous are the Megapodes that use mounds of decaying material to incubate their eggs. Less well understood is the Pelecaniformes' approach to nurturing their eggs. Pelicans, boobies and gannets partially cover their eggs with their feet and then crouch over them. For decades, this observation has invited speculation about the potential for heat transfer between the feet and eggs, but proper measurements had not been made.

Stephanie Morgan and her colleagues set out to end the speculation. Their main goal was to determine whether the bird's feet directly warm the eggs or if they merely pass on heat from the abdomen. The team studied nazca boobies at the Galápagos Island of Española. Nazca boobies, like all Pelecaniformes, have webbing between all four toes of their feet. During incubation, both parents take turns in wrapping the middle webs around the sides and top of their single egg. To measure heat transfer from the abdomen and the feet separately, the researchers took the booby eggs and temporarily replaced them with larger albatross eggs. The waved albatross' eggs are about twice as long and wide as boobies' eggs. So, the incubating parent was only able to cover the sides of the adopted egg with its feet, leaving the top of the egg uncovered by webbing and in direct contact with the abdomen. The egg was equipped with three temperature sensors, two on the sides where the feet touched the egg and one on the top where the abdomen covered the egg. In addition, Morgan and colleagues measured the soil and egg temperatures before the albatross egg was placed beneath the booby and after the egg was removed.

In all four trials, the incubating booby heated the albatross egg to above ambient and soil temperature. Temperatures at the foot–egg interface exceeded temperatures at the abdomen–egg interface, at least during part of each trial. And the feet of the booby were always warmer than the final temperature of their adopted egg. Temperature-sensitive paint showed that boobies' feet can be warmer than 40°C!

How do incubating boobies get such warm feet? The answer is in the blood. Foot-webbing is always vascularized, but incubating birds have a larger area of blood vessels than birds without eggs. More, or larger, vessels allow for a larger heat flow to the egg. As is often the case for abdominal brood patches in other bird species, female boobies had more vascularized feet than their mates, even though both sexes incubate.

This study ends the speculation about the role of warm feet in providing a cozy environment for developing embryos. Just like regular brood patches, the foot-surrogates are well vascularized and transfer heat from the parent to the egg. And, as usual, good science not only answers but poses new questions too. The next piece in the puzzle is an efficiency test: how do the feet compare with the abdomen for keeping their precious eggs warm?

Morgan, S. M., Ashley-Ross, M. A. and Anderson, D. J.(
). Foot-mediated incubation: nazca booby (Sula granti) feet as surrogate brood patches.
Physiol. Biochem. Zool.