From MN DNR eagles:"
Countdown to the hatchThe rare glimpses we get of our eagles’ eggs may suggest not much is going on with them, but inside those white shells a host of physiological changes are or will soon be unfolding in what amounts to a 72-hour countdown to the hatch.
One of the main things that needs to happen is for the embryonic chick to switch from getting oxygen through the membrane that surrounds it in the egg to breathing on its own. First, a muscle at the nape of the chick’s neck swells and causes the head and beak to twitch and press against an inner membrane that eventually is pierced, allowing air to reach the chick. At this point it starts to actually breathe, and the chick’s parents may be able to hear faint chirping from inside the egg.
As the chick breathes and re-breathes the air inside the egg, carbon dioxide accumulates, causing more muscle spasms. A bony protuberance on the chick’s beak known as the “egg tooth” taps at the shell until a tiny, barely visible star forms, letting in some fresh air. With that, the chick usually calms down for a few hours before creating a larger, visible hole known as a “pip.” EagleCam watchers may recall seeing the pip, with a tiny beak visible inside, in years past.
With an opening to the outside, the chick’s lungs become fully functional, which triggers the little bird to start rotating inside its shell, industriously cutting around the egg’s circumference until a cap is loosened and the bird first lifts its fluffy head into view. The week or so after hatching tends to be one of the riskiest for chicks.
These three eggs were laid on February 6, 9 and 12, and incubation usually lasts about 35 days, so we’re all keeping our fingers crossed and hoping for a successful hatch soon!"