A hen does not know if her eggs are fertilized or not. In fact (much like a human) a rooster can be infertile, so a hen’s eggs might not be fertilized even if she is in a flock with a rooster. Even more than that, a hen doesn’t know the difference between her eggs and eggs laid by another hen, even if the eggs look totally different and are a completely different color than her eggs. I routinely use one hen to hatch other hens’ eggs. I will even give a hen golf balls to sit on as placeholders if she’s ready to brood and I don’t have all the eggs I want to hatch gathered yet. Because a hen can’t tell if her eggs are fertile or not, I’ll tell you what a hen will do with any eggs she lays, fertile or not.
Many modern breeds and commercial hybrid hens will do nothing with their eggs other than lay them and walk away. A commercial hybrid isn’t anything scary, by the way, it’s simply a specialized cross-bred chicken, much like a Labradoodle or Puggle dog. Modern commercial breeds and hybrids have had the instinct to brood 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 and start laying eggs again. A broody hen sits on a nest (even if it’s empty) and doesn’t take good care of herself; she won’t eat much and doesn’t drink much, and can lose quite a bit of body weight and get in bad condition. She may not snap out of this trance if she doesn’t get triggered by feeling chicks hatching beneath her; chickens have no concept of how long it takes chicks to develop (21 days, by the way). Even before the advent of modern “factory” farming, farmers were trying to breed away the instinct for brooding chicks. This breeding has largely succeeded for many breeds.
This does not mean that no hens will brood eggs; many breeds still retain their instincts to mother. Silkies, for instance, are renown for their desire to sit on eggs. Other breeds such as Orpingtons, Brahmas, Cochins, Marans, Cornish, and others go broody quite regularly. I myself have a little Marans hen that was broody a month ago and hatched a clutch of chicks. 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+. 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.
There are very good reasons that she does not sit on the eggs from the beginning. Firstly, she needs to continue to eat and drink so that she doesn’t lose body condition and can continue to produce eggs for her clutch. Secondly, all the eggs need to begin developing on the same day. An egg does not start forming a chick as soon as it’s laid. Instead, the eggs are kind of in a state of suspended animation. You can’t even tell the difference between a fertilized and non-fertilized egg without cracking it and looking very closely. Once an egg is above about 98 degrees F for approximately 24 hours, however, it will begin to develop. This way, all the chicks start developing when the hen settles down to sit on them and are all developing at the same time, and none of them have a head start. Then all the chicks will hatch over a short period (usually less than 24 hours) and are all ready to venture out for food at about the same time. If some eggs started developing a week before the others (for example if the hen started sitting on them while she was still adding eggs to the clutch) then some chicks would be ready to hatch a full week before the ones that were in the last eggs to be laid. If this happens, the hen will abandon the not-fully-developed eggs and care for her already hatched chicks, and the not-ready eggs will die.
Two to three days after the first chick has hatched, the mama hen will come out of her broody trance and start to care for the chicks. In the meantime, the chicks will all stay under Mama and require no food or water; they are fed from the remnants of the yolk that is in their body for this purpose. Mama will care for them for a while–the exact time is different for each mother hen. Some care for them only until they are 12 weeks old, some will care for them longer.