Why do Sullies Swamp Hens (more mundanely known to the general public as 'moorhens' I believe) have a white butt? I mean, the rest of them is black, or blue, and they have this white tuft right on their posteriors underneath their (black) tailfeathers that looks like someone shoved a cockatoo up there or something.

Which might explain why there are so few cockatoos around these days...