поди ж пойми- попробуй пойми (try to understand) Сидит птица на попе. - it is joke Птицы не могут сидеть на стуле как люди. Birds can not sit on a chair, as people. Сидит птица на ветке - это значит она согнула ноги. (bend the legs)

поди ж пойми = literally "go figure" that out Сидит птица на попе. = well, the bird does sit with its 'butt' down on some surface. so it kind of sits on its butt. though, the word 'попа' is not really a 'butt', it's a most soft word for that part of the body, mostly used with a small children or when you wish to sound extremely harmless and soft. that part of the body has a lot of various ways to be named :).

Сидит птица на попе. Поп служитель культа, священник в церкви.