It's big mystery! I had a broody silkie hen on my covered back porch in a playpen that I normally use as a brooder. I had a 1/2 in. thick peice of plywood sitting on top of it. Being on my porch I was not to concerned about predators since my dog sleeps there and we have never had a problem in the last year or so we have been doing this, including baby chicks. There is a 4 in. gap on top on both sides but being very broody she had no interest in even getting off her nest, I even had to go out and pick her up off the nest for her to eat and poop. She was getting thin b/c she was just no eating enough. Today I cam home from church to check on my broodies (had three) to find her nest abandon and her dead in the yard. First of all she is a silkie and not sure she could even jump three feet to reach the top of the pen, she would have to be REALLY determined. Especially since she would have to squeeze with through the 4 in. gap. The plywood was still in place and not pushed back. As far as her body there where no wounds or broken skin. However, I know my dog may have had something to do with her being dead because she is notorious for pulling feathers out and there where feathers missing, not many but she was acting guilty about it. It's also possible if being broody, if she attacked the dog, she may have retaliated. She has been out all day with our other chickens and not hurt them, she likes to herd them. Ironically, my silkie run was left open this morning (not sure how, I was gone but my Mother was watching my nieces) and three other chickens where out roaming around. She had not been dead long when we found her this afternoon.