3/17/17 – Fans of the Hanover Bald Eagle‘s nest can celebrate this year’s St. Patrick’s Day with pip-watch! What is pip-watch? It is when people who watch nest cameras anxiously anticipate the coming pip, of course! What is a pip? Pip stands for ‘peep is pecking’ and it means an egg is beginning to hatch.
There are two forms of pip, the internal pip is when the chick breaks through the inner membrane, underneath the shell, and the chick can begin to breathe the air inside the shell. The external pip is when a small hole appears in the shell, and the chick can start to breathe air from outside the shell. The internal pip can occur 1-2 days before the external pip.
How do these pips occur? There are complex hormonal and chemical changes that occur that influence hatching, but simply put there are two main adaptations that assist the chick in hatching: the egg-tooth and the hatching-muscle.
The egg-tooth is a sharp projection on the top of the bird’s beak that breaks the inner membrane, and then the egg-tooth is scraped against the shell to create the hole that we can see in the shell. The egg-tooth falls off or is worn away within a few days after hatching. The hatching-muscle is a strong muscle on the back of the neck that helps the chick push its head back or up against the shell.
These processes take time. Once the external pip occurs it can take up to a day or more for hatching to be complete. The chick’s body has to adjust to the physiological changes that take place when it goes from getting nutrition from the yolk to breathing air. The yolk is also resorbed in this time. The respiratory and circulatory system need to adjust to these changes. Some of this time is spent resting and recovering from the exertion of breaking out of the shell.
Vocalizations may also occur while the eaglet is still inside the shell. Eaglets can be heard 12 hours or more before the pip occurs. For some types of birds these vocalizations serve to synchronize hatching of the clutch.
There are always exceptions to the rule! Did you know that kiwi, and ‘megapodes’ like the Australian Brush-turkey and NewGuinea Scrubfowl, use their feet, instead of an egg-tooth, to break out of their shell?