Survival Mode, Mountain/Forest Biome; I have a large enclosed yard area (wood fence) containing both my Chicken's and a small farm.

Chicken's are falling through blocks, at first I thought it was only one, I was concerned that it might suffocate so I attempted to dig it out, it fell further; acts like it's trying to swim through the blocks. So far none have died from the condition (when I was watching anyway) One may have gotten under the the fence in this manner.