Peek inside a bald eagle egg: 24 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 24.5 days old, the middle is 29 days old, and the oldest is 32 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 this animation uses a chicken instead of a bald eagle, the sequence of development is fairly similar.

Between 17 and 24 days:

  • Leg scales, tiny talons, and plantar food pads appear.
  • The eyelids are almost closed.
  • The eyes are no longer quite as large in proportion to the rest of the head.

We are 65% of the way to hatch! Other than the closing of the body cavity, most major morphological changes are done. From 24 days through hatch:

  • Eyelids will close all the way.
  • Eyes will grow into their sockets, more or less. Eaglets often have big bulgy ‘blueberry eyes’ when they hatch. Their eyes settle into their sockets during the first few days after hatch.
  • Natal down will grow from feather germs.
  • It will position itself for hatching.
  • It will internalize its egg yolk sac, sucking it into its body shortly before hatch.
  • Its body cavity will close (leaving behind an egg yolk sac scar).

So what happens right before hatch? Just a few days from external pip, the rapidly growing embryo is taking up nearly all the space in the egg. It…

  • Turns so that its head is at the large end of the egg next to the air space.
  • Pierces the internal membrane – the internal pip – and begins to breathe air with its lungs. Hatch has started!
  • Takes the yolk sac into its body as it consumes the remaining albumen and yolk.
  • Grows enough to contract the hatching muscle, pointing its head up and positioning its egg tooth against the shell of the egg. The eggshell is thinner and weaker than when it was laid, since the growing embryo absorbed calcium from the shell for its bones.
  • Rubs its egg tooth against the shell, which cuts a small hole. We have an external pip!
  • Rotates its body, slowly cutting a ring around the shell.
  • Pushes its body against the shell, forcing the shell apart.
  • Works itself free of the shell membranes and halves. The eaglet has landed and hatch is complete!

We are looking forward to hatch! Curious about what’s in store? Watch this 2015 video of the very final stages of hatch in N2!