We are on hatch watch at Decorah North starting tomorrow, March 23, when DNF’s first-laid egg turns 36 days old and the youngest turns 33. While both eaglets probably still have open body cavities, most of their major morphological changes are done. At this point:
- Their eyelids still need to close all the way.
- Their eyes are growing 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 is growing from feather germs.
- The chicks are squirming themselves into hatching position.
Hatch Timing
I’m very curious about hatch timing this year! Bald Eagles don’t delay incubation like Peregrine Falcons, but they tend to spend more time off their first egg if the weather is warm enough to allow delaying. DNF’s eggs and Mom’s first and second eggs have tended to hatch within roughly a day of one another, but D34 and D35, two of Mom and DM2’s offspring in 2020, hatched within six hours of one another. The weather was warm enough to allow for a little delaying this year. Will DN17 and DN18 hatch closer together? Will hatch number one be delayed a little bit? We don’t know for sure, but our camera operators and moderators have noticed that DNF is restless and appears to be listening to and looking at the eggs more often today. Go D17 and DN18!
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 – 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. Its body cavity seals, leaving behind a yolk sac scar, aka the eaglet belly button!
- 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 so looking forward to hatch! Curious about what’s in store? Watch this 2019 video showing pip in Decorah – and keep an ear out for egg talk! Shortly before hatch, eaglets begin vocalizing and parents softly vocalize back. Why? Researchers have suggested imprinting (eaglets hatch unable to see well, but able to hear well and see roughly, so vocalizations help to imprint them to parents), innate behavior (parents are unable to keep from responding to the vocalizations of their young), and encouragement (egg talk incites hatchlings to keep working at hatch). Whatever the reason, egg talk is lovely!
Information about embryo development hatch was taken from work done by Dr. Peter Sharpe from the Institute for Wildlife Studies. He developed a table of bald eagle embryonic development based on work done by Hamburger and Hamilton (1951). While the animation above uses a chicken instead of a bald eagle, the sequence of development is fairly similar.
The Raptor Resource Project