If you’ve been watching the eagles, their eggs and their new chick at EagleCam, you may have asked yourself this question: “If the feathers are there to insulate the eagles and prevent heat loss, how can a feathered bird keep its eggs and brood warm?”
The answer is that the eagle isn’t feathered at the part of its body that contacts the eggs and chicks. This involves a nice adaptation called the brood patch. The Stanford University bird webpage says this:
One of the main functions of the feathers is to insulate the bird — to prevent its body heat from being dissipated through the skin surface. Most birds have “solved” the dilemma posed by the need to both transfer and preserve heat by evolving “brood patches.” These are areas of skin on the belly that lose their feathers toward the end of the egg-laying period. In most birds the feathers are shed automatically, but geese and ducks pluck their brood patch and use the plucked feathers to make an insulating lining for their nests. The brood patch also develops a supplemental set of vessels that bring hot blood close to the surface of the skin. When birds return to the nest to resume incubating, they go through characteristic settling movements in order to bring the brood patch into contact with the eggs. In precocial birds, after the chicks have hatched the insulating feathers grow back. In passerines, and presumably other altricial birds, the regrowth of the feathers is delayed, and the patches remain functional through early brooding. Then they gradually disappear, restoring the adult’s thermoregulatory integrity about the time the young are fledged.
The placement of brood patches differs among groups of birds. There may be a single brood patch in the middle of the belly, as in hawks, pigeons, and most songbirds. Shorebirds, auks, and skuas have one on each side, and gulls and game birds combine these two patterns by having three brood patches. Pelicans, boobies, and gannets have none at all. They cradle the eggs in their webbed feet, cover them with the abdomen, and apparently warm them from both above and below.
When just one parent incubates, it alone develops a brood patch. If both parents incubate, both may grow brood patches, or one may cover the eggs without a patch, warming it less efficiently, but at least retarding heat and water loss from the egg.
Here’s a photo of an eagle showing its brood patch; from the Wildlife Center of Virginia.
And here’s a photo showing the position of the eagles’ nest and the webcam:
Be sure to check out the Hummingbirdcam too (you have to watch a brief commercial first). The mother bird (an Allen’s hummingbird) is constantly on and off the nest, frequently feeding her two young. It’s delightful—one of the best animal webcams ever!
h/t: Diane G