Laura Erickson's For the Birds

From earliest times, people have marveled at the egg and used it to symbolize new life, but few people recognize the equally miraculous adaptations parent birds have to ensure that their eggs will develop into real baby birds.

The babies of virtually all mammals develop inside the mother, where they can stay warm and nourished until they’re big enough to be born. Pregnant birds, and even nursing ones, would be too heavy to get off the ground, or would need relatively larger wings to support them, like bats, and so all in all eggs were just a better system. But because birds are warm-blooded, their eggs need much more attention and care than the eggs of reptiles.

Unfortunately, the feathers that make birds so wonderfully well insulated also hold the parents’ heat inside, away from the eggs, so birds developed what is called a brood patch—an area of bald skin on the underside where the incubating bird’s body is held against the eggs. The skin in that area is also supplied with an abnormally large blood supply, which makes it feel quite hot to the touch, like a hot water bottle, but few normal people go around feeling bird’s tummies to check this out. In those species where both parents incubate, both sexes develop a brood patch. In ones where just the female or just the male incubates, just the female or male develops a brood patch. In most species there’s one large, centrally-located brood patch, but some birds develop several smaller patches spaced to exactly match how the eggs are spaced in the nest. Most birds only have a brood patch during the time when they’re actually nesting—otherwise feathers grow in to cover it—but pigeons and doves have one year-round. The feathers fall out of their own accord, thanks to hormonal changes, in most species, but in ducks and geese the mother actually rips out her tummy feathers, which she then uses to line the nest, so the eggs will be well-insulated against the cold ground. This is especially important because the drakes never sit on eggs, and the mother would starve if she couldn’t occasionally leave the eggs unattended. She covers them with some of the feathers both to hide them and to keep them warmer until her return.

Parent birds instinctively turn their eggs occasionally. This prevents the developing baby from sticking to one area and becoming deformed or trapped in the egg, and also helps to make sure the baby is evenly warmed as it develops. As the baby gets close to hatching, the weight of the egg is asymmetrically distributed, and now when the parent turns it, the egg usually rolls back a bit, keeping the baby in the right position to be able to hatch out. Baby birds make tiny little noises starting a day or two before they hatch out, and parent birds are probably very aware of what’s going on, and sometimes even help the process by chipping away pieces of shell once the baby has broken through. It takes a lot of energy and patience to incubate eggs, but, as in humans, once the baby bird is born, the parents probably look back on the incubating process as a quiet, peaceful time in comparison to the hectic pace they’ll adopt while feeding and caring for those demanding little mouths.