Saturday, March 10, 2012


We are getting a lot of questions about eggs and hatching.  It takes 35-37 days for Bald eagle eggs to hatch, 24-28 days for Canada goose eggs to hatch, and about 28 days for great horned owl eggs to hatch. Despite the differences in incubation times, very similar things happen in the eggs of all three species. 

What happens within the egg shortly before hatching starts?

The rapidly developing embryo...
  • Grows large enough to take up nearly all the space.
  • Positions its body so that its head is at the large end of the egg next to the air space.
  • Begins to breathe with its lungs. Ever crack an egg and see the white membrane inside? Before the chick pokes its beak through this membrane into the air space, a special tissue called the CAM supplies oxygen to the developing embryo. Gases, including oxygen, leave and enter the egg by diffusing through the pores in its shell, across the outer and inner shell membranes, and into the blood in the capillaries of the CAM. From there, the blood circulates through the embryo and provides it with oxygen - no lungs required until the membrane is broken. 
  • Consumes most of the remaining albumen and yolk. When I was young, I thought that birds formed from the yolk. Not so! The yolk provides food and energy for the embryo.
The chart below outlines major developmental points in the lifecycle of a developing chicken embryo. At 20 days, the chicken is almost large enough to break the membrane and begin hatching. The timing would be a little different in the case of Bald eagles, Canada geese, and Great Horned owls, but the stages of development are the same.
Successive changes in the position of the chick embryo and its embryonic membranes. (From A. L. Romanoff, Cornell Rural School Leaflet, September, 1939.) (Fig. 9). Website:
So how do birds hatch?
The embryo has breached the membrane, is breathing air with its lungs, and is head up, with its head positioned at the large end of the shell.
  • Our embryo  uses its egg tooth, a small temporary structure on the op of its beak, to cut through the shell from inside. The eggshell is thinner and weaker than when it was laid, since the growing embryo absorbed calcium from the shell for its bones. The embryo rubs its egg tooth against the shell, which cuts a small hole.
  • As it rubs it rotates its body, slowly cutting a ring around the shell.
  • When the cut is complete, the hatchling bird pushes its body against the shell, forcing it apart.  It works itself free of the shell membranes and halves. Viola - a baby bird!
Here is a time-lapse video of a chicken hatching. The hatchling pips, or makes a hole in the shell, and begins rotating.

Altricial versus precocial
Our hatchlings face very different challenges. Canada geese are precocial - that is, they are relatively mature and mobile from the moment of birth. Eagles and owls are altricial, which means the young are helpless and require parental care. Bald eagle and Great Horned owl parents bring food into the nest for their young, often caching or storing prey for later consumption. This means that eaglets and owlets don't need to leave the nest or procure food until they fledge. The young goslings leave the nest within 24 hours of hatching and do not return to it. While their parents continue to provide protection and care, the goslings feed themselves.

The precocial goslings are also able to thermoregulate right away, unlike the eaglets and owlets. altricial birds require their parents (or a parent) to apply warmth until their down feathers are developed enough to insulate them. The eagle parents and mother owl will spend a great deal of time huddling over their young after they have hatched.


The following resources helped me write and understand this:


Cynthia (Iowa) said...

Thanks, Amy. Great information. Here is a video on avian embryonic development that goes well with your blog entry.

Anne said...

Are the terms altricial and precocial reversed in this paragraph?
The altricial goslings are also able to thermoregulate right away, unlike the eaglets and owlets. Precocial birds require their parents (or a parent) to apply warmth until their down feathers are developed enough to insulate them. The eagle parents and mother owl will spend a great deal of time huddling over their young after they have hatched.

beth92020 said...

Occasionally, when the eggs are turned by one of the adults, it appears as if one egg has a hole in it. Some feel it is just debris or soil but it really does look like a small hole about the size of a cherry pit. Have you noticed and if so, it that a fatal flaw?

Cindy said...

Awesome info, thanks so much.

Shar said...

That is fascinatng and thank you. I did not think about the difference between precocial and altricial.

Shar said...

Thank you -that is fascinating.

Grandmaj said...

Anne, I noticed this too. I hope we get an answer.

Brooke said...

I'm confused also...are bald eagles altricial or precocial? The paragraph referring to it is contradictory.

Raptor Fan said...

The first thing I noticed was the reversal of the terms "altricial and precocial" in the first and second paragraphs as did others. Being a proofer by trade, it did catch my eye. Otherwise, very informative.

Amy Ries said...

Anne, looks like I better correct it where it is wrong. Goslings are altricial, eaglets and owls, precocial.

Beth, we think that is debris or soil. We'll see what happens when the eggs hatch.

pvhdeerfield said...
This comment has been removed by the author.
pvhdeerfield said...

You have confused the terms altricial and precocial in this article.

Precocial comes from the same root as precocious, like a precocious child, advanced for its age. Precocial chicks, like ducks and geese, are hatched in a comparatively advanced state, generally fully-feathered and able to move around and feed themselves.

Altricial comes from a Latin root meaning "needing nourishment." Altricial birds like eagles and owls are hatched in a relatively helpless state, naked, eyes shut, and needing a lot of parental care and feeding.

Other than that, a very interesting article and blog. Thank you.

Grandmaj said...

Thanks for clarifying pvhdeerfield.

Jeanne said...

Amy, thanks for your factual information on birds. I am not a birder, never really liked birds until watching the WebCam for the first time last year. Now I am completely fascinated by the Bald Eagles. Your blog has helped answer my many bird questions and has given me the knowledge to explain to others when I share about the Bald Eagles.

krallison said...

So tells me the Bald Eagle egg takes two months to hatch. Who's right? Your webcam says about one month - they aren't hatching yet I see.

Totenzwerg said...

Actually, there are intermediate terms for birds which do not fit totally into one category or the other. Eagles, which hatch sighted and down-covered (characteristics of precocial) but unable to leave nest and/or self-feed (characteristics of altricial) are categorized as semi-altricial 1.

Totenzwerg said...
This comment has been removed by the author.
Lolihawk said...

Very helpful information. In the case of hawks - Harris hawks, e.g., which are larger than red-tails-is the time range similar for eggs to hatch?

Sian Min said...

This is a fantastic blog. So much educational information. I've been drawn to the eagles (and by extension, other birds) for just over a year. I used to take them for granted, though I always thought they are amazing.
Thanks, Amy, and RRP.