Here's just a theory: The sound of chicks getting a hen to want to brood. Just a theory though.
Also, something to think about... Is it it's hormones, if there's something that you can give your hen to increase those?
An incubator is cheap if you need baby chicks that badly. By the time you pay some scientist to prove it and then the hormone therapy for a bird I could have hatched out a few dozen flocks.