Saturday 14 March 2015
A hen does not know if her eggs are fertilised or not. In fact (much like a human) a rooster can be infertile, so a hen's eggs might not be fertilised even if she is in a flock with a rooster.
Many modern breeds and commercial hybrid hens will do nothing with their eggs other than lay them and walk away. Many have had the instinct to brood [sit on their eggs to hatch them] bred out of them over generations. In a modern egg production facility, you do not want a hen to "go broody". When hens are ready to raise chicks, they will stop laying eggs for that period and it's very hard to convince them to give up the idea.
This does not mean that no hens will brood eggs; many breeds still retain their instincts to mother. Silkies, for instance, are renowned for their desire to sit on eggs. Other breeds, such as Orpingtons, Brahmas, Cochins, Marans, Cornish, and others go broody quite regularly. When a hen that has broody instincts lays an egg, she is forming a 'clutch' of eggs. She does nothing to care for these eggs other than hide them in a secure place until she is ready to sit on them. She will continue to lay eggs in this clutch until she has 'enough', which is a number anywhere from seven to as high as 20-plus. Once there are 'enough' eggs, a hormonal switch will occur that will put her into what's best described as a broody trance. She will stop laying eggs and begin to sit on them instead.