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: