Bald Eagle

Bald Eagle Nesting Phenology

In order to have Bald Eagles in the future, Bald Eagles living today have to reproduce. This involves a LOT of critical steps, and scientists are still trying to figure out a lot of the details.
Because of the huge variation in the timing of breeding in North America, it's impossible to make a phenology chart with the precise timing for every place. But the steps involved in nesting have to happen in the same order.

Eagle family and nest

• Establish territory and build nest (1-3 months).
• Egg Laying
• Incubating Eggs
• Taking Care of Nestlings
• Taking Care of Fledglings

Territory Defense and Nest Building: 1 - 3 months
Eagles are tricky to study because the male and female look so much alike, and because they are so wary of humans near their nest. Some pairs may approach nesting in a different way than others, but it's hard to be sure. Scientists have observed some mating behaviors on wintering grounds, but have also seen some males and females start courting in breeding areas. Females seem to be the ones who choose the territory and the nesting tree, because when a female loses her mate, she usually very quickly attracts a new male, even if she's already sitting on eggs fertilized by her first mate.

Both the male and female bring nesting materials; some eagle researchers believe it's the female who usually places the sticks in the nest. It takes from 1 - 3 months to build an eagle nest. Once it's built, a pair often uses it year after year, adding sticks and fresh plants and working on construction at the beginning of the nesting season. Also, many times the pair makes nest repairs or builds onto the nest for a while after the babies have left for the season.

Both the male and female defend their territory against possible predators, especially ravens and other raptors. In defending the territory against other Bald Eagles, usually males chase off other males, and females chase off other females.

Egg Laying: 3 - 6 days
The female lays 1 - 3 eggs--most often 2. She doesn't usually lay one egg each day, so it usually takes her 3 - 6 days to complete her clutch. Each egg weighs roughly 110 - 130 grams. Adult females weigh about 4.6 - 6.4 kilograms.

Incubating Eggs: 35 days
As soon as the first egg is laid, the female and the male take turns incubating. Both male and female eagles form a brood patch--a bare spot on their tummy where they can press their hot skin directly against the eggs or chicks to keep them warm. The female's brood patch is a little bigger and more feather-free than the male's. And the female incubates the eggs more often than the male does.

One study showed that the female was responsible for 72% of the incubation. (How much was the male responsible for?)

Eagles sit on their eggs most of the time--one study showed that the eggs were incubated 98% of the time! But when the temperature is warm and there is little wind, the parents incubate less often. Sometimes when the parents leave the eggs, they cover them with feathers and nesting materials. Scientists don't know for sure whether they do this to keep the eggs warm or to hide the eggs so predators don't steal them.

Eagles have VERY sharp claws on their powerful talons. When the incubating parent is moving about the nest, it often clenches its talons so the sharp claws can't hurt the eggs or babies by accident. The parents are also very careful to step around the eggs to avoid crushing them. Parents probably turn the eggs at least once a day, but scientists aren't sure how often this happens or which parent does it.

Incubating lasts about 35 days.

Hungry nestling Credit: L T Bascher

Dinner is served! Credit: Ray Foster

Nestlings Credit: Peter Nye

Some scientists did a study keeping track of all the time that the parent eagles spent at the nest. Once the babies hatched, the female was present at the nest about 90% of the time. The male was present about 50% of the time. This adds up to more than 100% because sometimes BOTH parents are present at the nest. During the study, at least one of the parents was at the nest almost all the time.

When the weather is hot and sunny, one or both of the parents shade the babies. During the first four weeks after hatching, one of the parents (usually the female) broods the babies to keep them warm almost all the time, especially when the weather is cool. At this time, females brood about 65% of the time and males about 35%.

In one study, the parents brought food to the nest 1 - 8 times per day. The average was about 4 times a day. During the first two weeks, the male provides most of the food. After 3 or 4 weeks, the female provides as much food as the male, and by the late nesting period, the female provides most of the food.

As the babies eat, poop, and grow, the nest starts getting dirty. The babies usually lean over the edge of the nest to poop, but once in a while they miss! And food particles remain in the nest after each feeding. After a while, these build up to a goopy, stinky mess. Eagles simply do not have any idea about cleaning their nest. Instead, they add fresh leaves and other plant parts to cover up the mess. (This is a lot like sweeping dirt under the rug or hiding dirty clothes under your bed!)

The young practice flapping for weeks before their first flight. Sometimes one of them falls from the nest. If this happens, it can't usually get back into the nest. But the parents usually feed it on the ground. Sometimes when the babies remain in the nest for 11 or 12 weeks, the parents seem to encourage the babies to fledge by flying around the nest carrying food.

Fledglings are big birds!

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