Peek inside a bald eagle egg: 17 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, there are two eagle eggs in Decorah. We’re not sure whether the oldest or second oldest egg cracked, but we do know that the youngest is about 17.5 days 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 11th and 17th day? At eleven days, our embryonic eagle had a head, the beginnings of an eye, stiff differentiated limbs, the beginnings of a beak, rudimentary digestive organs, and a defined sex.

A chicken embryo roughly halfway to hatch

Chicken embryo 50% of the way to hatch

Between eleven and seventeen days:

  • The egg tooth and two scleral papillae form on the 11th-12th day. Scleral papillae are the precursors to scleral ossicles, or bones within the cornea of the eye. Scleral ossicles form a bony plate around a bird’s eye, giving it a rigid shape and forming a point of attachment for the ciliary muscles that control focusing.
  • Limbs bend as they grow longer and become more defined.
  • Feather germs begin forming. Dorsal feather germs form around the 12th day, followed by ventral and flight feather germs.
  • The nictitating membrane becomes visible on day 13 and eyelids begin closing.

At 17.5 days, we are roughly halfway to hatch. Our embryo’s head is disproportionately large, but it is definitely a bird. It has a beak, distinct toes, bent limbs, and eyes that take up most of its head. But its eyes and eye cavities aren’t done forming and it needs to develop scales, nails, rough pads and spicules, and down feathers. Its yolk sac and small intestines are still outside its body cavity, and it has a lot of growing to do!

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: