I was thirty before I heard someone use the word "butt" to refer to an arse-end. Arse was the word I knew, and still use.

Nowadays, more than forty years later, I hear that this bird is known informally in the USA as a "butter-butt." I grew up eating more margarine than butter so both parts of the name strike me as odd.

I propose to you that we call these Yellow-rumped warblers marge-arses.