Once an eagle pair has their nest prepared for eggs, they will continue to modify the nest by lining the “cup” with grasses and adding sticks to reinforce the rim. As egg-laying time approaches, the female will spend more time on the nest as the male searches for a few more branches and grass, and occasionally brings back a snack for the female, such as a fish. The female begins to incubate when the first egg is laid and she will lay another egg two to four days later. Sometimes a third egg is produced in another few days.
The purpose of spaced egg laying is to stagger the hatching times. This results in size differences in the eaglets when each one hatches 35 days later. It’s nature’s way of ensuring that at least one eaglet survives to fledgling age. If food is scarce, then the older and larger one gets fed first, as it’s more aggressive in taking food from the parent’s beak. The younger and smaller eaglet is not powerful enough to fight for food and has to wait until the older one has eaten. In some cases, it is known that the older eaglet will kill the younger one and eat it when there’s not enough food to go around.
If the initial clutch is lost early, a second clutch might be produced.
Both adults share in incubation duty, but the female does the major part of the job (50% to 75% of the time) with the male giving her breaks to hunt, preen or exercise. He will often bring food to her while she is incubating and gives general protection to her and the nest area from predators. Both adults develop a brood patch, a featherless area on their bellies, to allow direct contact and thus heat transfer to the eggs. The eggs are regularly turned (about every hour) to ensure even heating and prevent the embryo from sticking to the shell. I have noticed when a male returns to the incubating female, he will often bring a piece of food or another stick; it seems like a peace offering from the one who has been away for so long!
Development of the egg progresses with the start of limb buds, eyes and beak. During the next stage (11 to 17 days), the embryo begins to take on a recognizable bird shape and develop a sharp point (egg tooth) on its beak that will eventually be used to break free of the egg’s shell. After that stage, the eaglet grows to fill all of the space in the shell and absorbs calcium from the shell to strengthen its bones — which at the same time makes the shell thinner and easier to break when it comes time for hatching on the 35th day. About 2 to 4 days before that, the eaglet uses its egg tooth to peck a small hole in the egg shell, and eventually a circular ring around the inside of the shell, until it can push the shell apart and emerge.
In the first 24 to 48 hours, the eaglet uses the yolk that was drawn into its body to sustain itself. The whole hatching process takes up to 48 hours to complete.
Once hatched, eaglets are fed small pieces of fish or meat by their parents. In the first few weeks, the male provides most of the food while the mother feeds and provides warmth for the little ones. About the fourth week, an eaglet’s light gray down is starting to be replaced with juvenile feathers, and by the seventh week, the dark feathers cover its body. At this point, an eaglet can stand, tear apart its own food and exercise its wings by flapping them.
At eight to 10 weeks, eaglets begin to move around the edge of the nest and onto nearby branches. They will also begin to “hover,” actually rising off and a bit above the nest. These exercises help to build strength for their first flights when they are 10 to 14 weeks old. They often return to the nest for food and rely on their parents for several weeks after fledgling from the nest.
Next comes the difficult stage: learning how to hunt and survive the winter, which is the most dangerous time for young eagles because food sources are less abundant and there is strong domination of food by adults. If they make it through that first winter, it’ll be a five-year process to develop that white head and tail that mark the bald eagle as a symbol of our country.