Peek inside a bald eagle egg: 11 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 11 days, 10 hours old, the second egg is seven days, 14 hours old, and the third egg is three days, 13 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

What happens between the third and the 11th day? When we last touched on the topic, our embryonic eagle had inner and outer layers, a developing circulatory system, a brand new heart, and an anterior-to-posterior template in place. It looked more like a tadpole than an eagle, but the stage was set for it to begin developing limbs, a head, and eyes.

Chicken embryos in the same stage

Chicken embryos 23 to 25% of the way through development: roughly equivalent to an eagle at nine days of age. 

Between 3.5 to nine days:

  • Extra-embryonic membranes seal around the developing embryo. The embryo itself is surrounded by an amniotic membrane, which cushions it and keeps it properly hydrated. The amnion is surrounded by an allantoic membrane, or allantois, which stores nitrogenous wastes produced by the embryo. The allantois is surrounded by a chorionic membrane, or chorion, which facilitates the exchange of oxygen and carbon dioxide between the embryo and the egg’s external environment. These three membranes form a sort of womb that supports the embryo as it grows and develops.
  • Wing, tail, and leg buds form.
  • Eyes begin to form and develop pigment.
  • The neck thins, clearly separating the embryo’s head from its body.
  • Leg buds grow larger than wing buds and limbs begin to form.
  • Elbow and knee joints are distinct by roughly 8 days and digital grooves – the precursors of toes and wing structures – are distinct by roughly nine days.
  • The embryo’s beak and tongue begin to form.

At nine days, the tiny embryonic eagle is about 25% of the way to hatch and still doesn’t look especially birdlike. It has a head, the beginnings of an eye, stiff differentiated limbs, the beginnings of a beak, rudimentary digestive organs, and a defined sex. The stage is set for further organization. Next up? An egg tooth, true eyes, and feather germs.

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: