Like any other animal, chickens have natural instincts. For chickens, broodiness is one of them. When a hen gets broody, she is overcome with a desire to sit on her eggs until they hatch. Although this can be good news if you're hoping to increase your flock, it can also be an unwelcome development.
When a hen is broody, she will stop laying until the egg she's sitting on hatches, which can throw a kink into your egg-production plans. A broody hen will often neglect her own health by not leaving the nest for food or water, which also makes her susceptible to parasitic infestations. To make matters worse, broodiness is contagious, and the longer one hen does it, the more likely the other hens will catch the fever.
Steps can be taken to discourage broodiness, but some breeds of chicken are more prone to it. If you want to avoid this quality, steer clear of broody-prone breeds such as the Silkie (pictured). On the other hand, some chicken farmers will use a Silkie's motherly inclinations to their advantage. By placing fertilized eggs of other hens under a broody hen like a Silkie, this frees up the other hens to continue producing eggs while the Silkie plays foster mother.