Peek inside a bald eagle egg: 4 days!

As of this writing, we’re still waiting for eggs at Fort St. Vrain. The first Decorah North egg is seven days old and the second is four days old.

What do embryonic eagles look like as they grow and develop 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

After fertilization

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

As the fertilized egg begins its journey through Mom’s reproductive system, sperm and egg unite to form a single cell. That cell divides to produce multiple cells, which organize into layers. Locations for a head and tail are established, the emerging embryo divides into blocks called somites, and basic life support structures start emerging, including the skin, nervous system, circulatory system, gastrointestinal system, and optical system. The embryo turns onto its left side and 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.

People sometimes worry when they see blood on the eggs. It looks alarming, but the embryonic eagle is not vascularized enough to produce substantial amounts of blood and may not even have a heart. At four days, the egg is mostly yolk and albumen, so damage will appear as a crack and leak of clear or yellowish fluid. Eagle parents often have messy lunches and sometimes a little bit gets left behind on their eggs!

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: