A hen that is committed to hatching chicks is known as a broody. The state of being broody is controlled by instinct, hormones and lighting conditions. Left to her own devices, a broody will lay a clutch of eggs, then stop egg-laying and sit on them for 21 days (more or less) until they hatch.
During the last couple of days we wondered why we only got 4 eggs from our 5 hens every day. Until we saw a well hidden nest inside a stack of branches with one of the hens sitting on. It turned out that she is sitting on more than a dozen eggs!