Designated Rooster?

I've heard that if there's no rooster a hen will take it's place. Is this true? Because the other night my dad, my brother, and I went in to put in the hens and one was standing in the run doorway. Now when we went to close the door the hen would jump in and out. I thought it could be guarding the flock. So we closed the door and left the chicken outside just for a few minutes. So we went in the coop to check for eggs and refill the feeder. And when we went to check the "guard" it was scratching at the door and it wanted IN! We used to have a rooster but we gave him away.