Chickens (ducks too) also have the ability to sense a dud egg, and they will move them out of their nesting area. We just had a broody hen hatch a bunch, and a broody duck about to hatch a bunch... and they both rolled a couple eggs out from under them. Sure enough, when we candled them, they were duds.
Also... We've also caught crows in the Coop. They're smart birds... They've stolen eggs from us before and don't seem to give up. But we don't mind them, as they help keep the Hawks at bay.