This is one of the baby woodpeckers. He/she has finally got the idea that he/she needs to hang on to the feeder, but frequently sits on the post and lets mother feed him/her. After a while she gets fed up with this and pushes him off the post!
I have noticed with all the birds (not only the woodpeckers) that visit, that the skinny harassed-looking ones are the parents, and the lazy round ones are the children……………….
P.S. – apologies for the quality of the photo, it was taken through the window which obviously needs cleaning!