Henhouse Update

Petronella, our first hen, has now been sitting on her eggs for two weeks. Citronella, another hen, took over the adjacent nest and has been faithfully sitting on eggs as well. Both have been faithful to their task and seldom leave their nests. Petronella, though, apparently needed a break and spent most of Saturday running around the yard with the rooster and two other younger hens. Citronella for some reason moved from her own nest and sat on Petronella's eggs all day, leaving her own to grow cold. Petronella decided to come back late in the afternoon. I'm not sure what happened, but for some time Citronella refused to move and the two hens were sitting in the same nest. By evening Citronella had returned to her own nest and the two hens were back in their rightful places. I'd love to know the chicken psychology of what's happening with these two. Any experienced chicken farmers who are out there, please enlighten us!