If a crow tries to fly over a block in order to pick up an item or put it in a chest, instead of flying over the block like it would logically do, it instead tries to fly through the block and gets stuck. This is likely because fences are unusual.

I replicated the glitch in this image. I initially thought it was just fences, but as I tested more I found out it cannot pathfind around any block if it's trying to pick up an item.