Eastern Bluebird eggs - external pipping evident, newborn chick in back. Photo by David Kinneer.
If the egg is incubated, the embryo inside develops. The parent(s) periodically rotates and rearranges eggs in a nest to ensure equal heating. This also prevents the shell membrane from adhering to the shell, which could interfere with hatching.
First, circulatory, digestive and respiratory, and nervous systems start to differentiate.
Then limbs may begin to move.
An egg tooth is formed to help the bird break out of the shell when it hatches.
The yolk sac gets smaller. In preparation for hatching, the embryo is in a tucked position. It uses its bill to puncture the innermost shell membrane at the blunt end of the egg (called internal pipping.)
The embryo begins to breathe the pocket of air between the inner and outer shell membrane.
After a few more hours, the egg tooth is used to break through the outer shell membrane and the shell (called external pipping - see cracks and holes in photo above.)
When it penetrates the shell, the hatchling emerges from the egg.
The parents generally do not help the young get out of the shell, although they may watch intently. They may remove or eat the egg shell after hatching.
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