Turning The Eggs

By Doug Pifer

It’s eagle nesting season again. The bald eagles at the National Conservation Training Center (NCTC) in Shepherdstown have laid two eggs and incubation has begun.

Since 2015, we have joined the hundreds of regular viewers of the NCTC live nest camera. It’s very easy to become hooked on the daily drama. As the introduction to the site says, this is a live camera in real time, and anything can happen. This year, however, the nesting season has progressed relatively calmly for the local pair of nesting eagles. One or another of the pair sets on the nest day and night, trading off in shifts. From time to time, the parent eagle will stand up and carefully turn the eggs.

Anyone who has used an incubator to hatch hens’ eggs understands that egg rotation is necessary so the eggs can develop and hatch properly. Having watched many birds, wild and domestic, turn their eggs, I believed they did so to keep the eggs uniformly warm. But there’s so much more to this fascinating story.

Inside the egg, the embryo of the developing chick is surrounded by the egg white and the egg yolk. Egg yolk, encased in a membranous sack attached to the abdomen of the developing chick, is largely composed of fat. Because it is lighter in weight than the egg white, the yolk always floats to the surface inside the egg. The egg must be turned occasionally because if it remains in one position for too long, the yolk is likely to dry out and  stick to the membrane that lines the eggshell, causing the developing chick to die.

How could an egg dry out so easily? The shell of a bird’s egg looks solid but is really  porous, so the developing chick can get all its essential oxygen and humidity from the surrounding air. When a bird turns the egg, it exposes fresh surfaces, allowing more oxygen and humid air to circulate inside the egg.

Normally eagles turn their eggs approximately once every hour. The parent eagle stands up and carefully steps back from the eggs, curling its talons into balled “fists” to keep them from damaging the eggs. Then, using its bill, it gently turns the eggs. The eagle immediately settles back onto the eggs. The bird rocks from side to  side (which my wife calls the Mama Rumba) so that an area of warm, bare skin on its belly, called the brood patch, touches directly against the eggs. Both male and female eagles have a brood patch, surrounded by dense body feathers  that help to insulate the eggs from the cold.

Many birds wait until all the eggs in a clutch are laid before they begin to incubate, so  all the eggs hatch around the same time. Winter nesting birds like bald eagles cannot afford to wait because their eggs could freeze. Typically, the first bald eagle egg appears around the second week of February. They may lay as many as four eggs, although two eggs are usual. Generally, the eggs are laid two or three days apart. The Shepherdstown eagles laid their first egg this year on February 10, and their second on February 13. A third egg was laid February 17.

What’s so exciting about watching a bird sitting on eggs? Tune in for a while and you’ll see. At any time, an approaching eagle, raven, or a night marauding owl or raccoon creates tension and drama for  the incubating parent bird. Winter storms rock the nest tree while snow and ice cover the nest and the setting parent bird. Heavy rain soaks the feathers of an incubating eagle as it flattens itself with wings  spread in a “mumbrella” to cover the eggs. Mom or dad may be late returning with food, or one of them might come in late to take over their shift. Sometimes both parents leave the nest unattended. 

What then?

If you see the eggs lying exposed with no parents around, don’t worry. Even in  the dead of winter, the eggs can overheat. Consequently, the eagles occasionally leave the nest, usually for no more than twenty minutes, to allow the eggs to cool down. Visit the link for an article referenced on the NCT eagle nest chat line at https://birdwatchingpro.com/why-do-eagles-roll-their-eggs/ to read more about the fascinating subject of incubation. Visit the eagle cam at https://www.fws.gov/nctc/eagle-cam; and click on the link to the cam.