One of our hens (they are all about 9 months old) is acting strangely. She is wanting to hunker down in the coop, even while the other two are free ranging the yard. She is obsessing over her laid eggs and warming them as if they were fertilized. We have no roosters. She gets very defensive and puffs her feathers when I try to move her. I'm trying to break her out of it by removing the eggs, picking her up and out of the coop to get her out and about. Any advice or explanation of what might be causing this behavior?