Peek inside a bald eagle egg: 4 days!

This blog was first published on March 23, 2017. We reposted it to give everyone a peek inside the eggs. As of this writing, the first Decorah egg is 3 days, 12 hours old, the first Decorah North egg is 4 days, 19 hours old, and the second Decorah North egg is 1 day, 22 hours old. 

What do embyronic eagles look they look like as they develop and grow inside their eggs? Dr. Peter Sharpe from the Institute for Wildlife Studies developed a table of bald eagle embryonic development based on work done by Hamburger and Hamilton (1951). While not all bald eagle eggs hatch in 35 days, the stages of development look something like this…

Development of an avian embryo

Development of a chick, drawing from Frank Lillie photos. Artist William Sillin

Chicken embryo at roughly two days incubation: equivalent to an eagle at about 3.5 days

So what happens in the first four days? As the fertilized egg begins its journey, a single cell is formed by the union of sperm and egg. It divides into multiple cells and forms layers. Locations for a head and tail are established, the emerging embryo divides into blocks called somites, and basic life support structures begin to develop, including the nervous system, skin, circulatory system, gastrointestinal system, and optical system. The embryo turns onto its left side. Its heart begins to beat roughly 72 hours after incubation begins.

At four days of age, the embryonic eagle doesn’t look anything like a bird, but it has inside and outside layers, it can transport materials through its developing circulatory system, and its nervous system has an anterior-to-posterior template in place. The brain and nervous system will continue to grow and change, but the stage is set for the development of a skeletal system, limbs, a beak and tongue, foot and wing digits, and organs.

A lot of people expressed concern about a bloody egg in Decorah on February 25. While it looked quite alarming, the embryonic eagle is not nearly vascularized enough to produce substantial amounts of blood. At the time we saw the messy egg, the developing eaglet was extremely small and had not yet developed a heart. The contents of the egg were primarily yolk and albumen, so damage would appear not in the form of blood, but as a crack and leak of clear or yellowish fluid. Mom had a bit of a messy and very fresh lunch, and left some behind on the egg when she rolled it.

Illustrations were taken from Popular Science Monthly/Volume 71/September 1907/The Problem of Age, Growth and Death III: Link. Thanks to artist William Sillin for allowing us to use his lovely illustrations: (check it out – his illustrations are very cool!). Also take a look at this cool plate by Keibel and these lovely photos of chicken embryos:

Things that helped me learn about this subject: