Eagles are among the most powerful birds of prey in the world. They typically belong to the accipiter family, also known as “Accipitridae”. The main difference between them and hawks is that eagles are much larger and have a wider wingspan.
When it comes to egg laying and incubation, eagles are known for their unique standards, such as nests very high up the trees. If you’ve watched an eagle while incubating an egg, you’ll also notice that they turn the egg from time to time, so why do eagles roll their eggs?
There are three main reasons why eagles roll their eggs. The main one is to keep the yolk centered in the middle and prevent it from sticking to the inner membranes of the egg. The other reason is to distribute the warmth as well as nutrients evenly across the egg for healthy and balanced growth.
If you want to find out more about egg rolling as well as the incubation of eagle eggs, this article will satisfy your curiosity with answers to the most frequently asked questions. Let’s dive right in!
Although rolling the eggs might look like a bizarre behavior by eagles, it’s actually an integral part of the health of the chicks and the development of the eggs during the incubation period.
While there are plenty of theories about egg rolling during incubation, three explanations seem to be the most widely accepted by the scientific community, so let’s have a look at each one of them:
To have a better understanding of this point, let’s have a quick look at the anatomy of a fertilized bird egg.
Inside the shell, there’s an egg yolk, Chalaza (the first stage of the embryo), and the albumen protein (the egg white), all enclosed inside a thin membrane. Unlike what many people think, the yolk isn’t actually the embryo but its food.
Top Tip: Since the egg yolk is mostly fatty acids, it has a lower density than the albumen, which leads it to float up slowly inside the egg.
If the egg yolk slowly rises and reaches the surface, it can stick to the egg’s membrane, which blocks the fine blood capillaries and vessels that feed the embryo, which can be fatal for the developing chick.
For that reason, rolling the eggs on other sides is quite essential to keep the yolk floating in the center.
You need to know that most bird eggs typically can’t develop or survive if their temperature falls below 34 degrees C (93 degrees F) or exceeds 40 degrees C (104 degrees).
In other words, the temperature of the eggs must be kept at a relatively warm temperature that is close to the internal temperature of most mammals and birds.
For that reason, birds sit on their eggs to keep them warm and protect them from predators, which is known as “incubation”, and eagles are no exception to this rule.
Since warmth is important for the embryo to develop, the eagle will roll the eggs every now and then in order to keep both sides of the egg equally warm.
Eagles remove some of the feathers at the breast area to expose the skin, which helps in transferring body heat to the eggs at a much higher efficiency. This part is known as the “brood patch”.
Since the nutrients inside the egg are separated, turning and rotating the eggs will also help the embryo to absorb as many nutrients as possible.
Top Tip: This is especially important in the case of albumen, which contains an essential amino acid called “albumin”.
According to some research, when a bird chick doesn’t get enough albumin during its incubation time, it reduces its survival rate significantly after hatching.
Ideally, eagles will sit on the incubating egg for most of the day. To do this, both parents alternate and take turns to sit on the eggs for some time, although the female sits for a much longer time than the male because it has a larger brood patch.
During that period, the parent eagle will roll the eggs once every 1 to 2 hours, depending on the warmth of the egg.
To do that, the eagle will stand up and back off the eggs, after that, it’ll use its curved peak in order to gently turn or nudge the egg on the side.
During that time, the eagle will curve its sharp talons into a ball in order to avoid puncturing the eggs or damaging them.
Almost all birds use the egg rolling technique for the previously mentioned reasons. However, the duration at which an egg can stay on one side may vary from one type of bird to another.
This makes some birds roll their eggs every one to two hours like eagles and hawks while other species of birds will roll their eggs once every several hours, making them look like they don’t roll their eggs.
The difference here depends on the ambient temperature where the chicks are being developed as well as the density of the yolk to albumen in each specific species.
Eagle eggs are quite similar to chicken eggs in terms of general shape. The eggs are typically oval and slightly large, measuring around 7 to 9 cm in length (2.7 to 3.5 inches) and about 5.5 to 5.7 cm wide (2 to 2.2 inches).
The eggs come in various shades of off-white to buff color and often have several dark specks all over the egg.
Author Note: As for the nest, eagles usually build relatively large nests when compared to other birds. They also look for the highest tree with several branches to build their nests.
Keep in mind that eagles have a strong affinity towards their nest, so migrating eagles will usually return to their nest in the following year.
The short and comforting answer to this question is no. Eagles and other birds have been rolling their eggs for hundreds of thousands of years and it’s critical for the well-being of the chicks.
The bedding of the nest where the incubation takes place is usually soft, thanks to the small twigs and leaves used.
Even if there are cracks showing up in the eggs, it’s rarely due to rolling eggs. Instead, it’s caused by the pipping of the fully developed chicks that are about to hatch.
Although the eggshell is solid and relatively hard in the case of eagles, it’s porous enough for oxygen to diffuse through its microscopic pores.
The oxygen then enriches the blood vessels that are connected to the egg membrane, which is passed all the way into the bloodstream of the chick.
Carbon dioxide is also exchanged through other blood vessels that run separately and in the opposite direction.
After successful mating and fertilization, the female bird lays its first egg in about 5 to 10 days. The incubation period begins with the first egg, even if the female eagle may lay a second egg in the following few days.
Author Note: In some cases, and depending on the fertility and nutrition of the eagle, it may lay a third egg as well. The group of eggs that are laid within the same incubation period is known as the “clutch”.
The incubation period of a healthy eagle usually takes anywhere between 34 to 36 days, so the average time for eagles to hatch is 35 days.
As previously mentioned, the female eagle lays its eggs on separate days, so the first chick is usually noticeably older than the third one because of the quick development during the first few days.
Eagles don’t have a specific method to figure out whether one of their chicks has died before hatching out of their egg.
For that reason, if an egg fails to hatch for the first few days, both parents will continue sitting on the egg to keep it warm while exchanging turns. After that, they’ll start to spend less time sitting and incubating the egg.
After about a few more days, if the egg doesn’t hatch, they’ll eventually abandon the egg and give up trying.
This wraps it up for today’s guide that shows you why eagles roll their eggs and how this helps in the incubation process to produce healthy chicks.
As you can see, the rolling is mainly done to prevent the light yolk from floating up and sticking to the egg membrane, which kills the embryo. Also, it provides the eggs with balanced warmth on either side.
The action doesn’t inflict any damage on the eggs because it’s only a single roll and usually happens on the smooth nest bedding.