Chicken Wiki
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A broody Silkie, incubating her clutch.

Broodiness is a natural tendency for a hen to sit on a clutch of eggs to incubate them, in attempt for the eggs to hatch into chicks. They often portray many different behaviors, and will not often get off of their nest, unless getting food or water. This process last for around 21 days, in which if the incubation process was successful, the chicks will hatch.  

In some instances, another hen will incubate eggs that are not her own if the original mother has died.