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 :).