Not all hens will go broody. Selective breeding by humans has removed the broody instinct from most popular large fowl breeds because we want them to keep laying year round, not take a couple of months off to raise chicks. Some breeds are known for being good brooders, like silkies or many of the bantam breeds--Old English Game, for instance, are good broodies usually. There is really no way to make a non-broody hen broody. If one is broody, she will be so on her own timing, and regardless of whether she has any eggs to sit on or not. So, if you don't have a broody hen, or you want to hatch eggs at a time she doesn't want to sit, you will need an incubator.
