While Tim is a city boy who may not have spent much time around actual robins, he still recognizes one when he hears and sees it, and the fact that it lands specifically on his shoulder indicates to him that this is probably one of his friends being ridiculous. Of course, in this shape, he can't tell which one. Probably one with a terrible sense of humor.

So really, it could be any of them.

He raises an eyebrow at the robin. "Can I help you?"