I think it's the mouth, more specifically the idea of a bird person blow?job.
I mean, there aren't any lips, so that really puts a damper on the whole 'blow' thing. i guess we're kinda stuck with lots of crazy bird tongue and beak stuff, but hey that's cool too, i'm an equal oppurtunity employer. i'd probably start out nice and slow and sensual with some mood music, probably charlie parker for good measure. make a little pile of bird seed in my bare navel and let them pick it out, grain by grain by grain. then i'd drizzle a liitle bit of hummingbird food on my d for encouragement and whisper 'polly want some cummies?' of course the answer is a horrendous 'aaayykk' which obviously means something like yes. i'd tease it for a minute, smack it across the beak a couple times, get it really hungry. then while the early bird is getting your worm, just as the sensation starts to level off a little bit, i'd have em tickle the underside of my sack with one of their flight feathers, maybe the poop chute with their other wing. when you get to the edge, grab em by the crest on their heads and make em really deep throat it while I bust one up in their gizzards and honk like a retarded goose. really i'm most fascinated i think about the sword of dumbassocless that is their beak wrapped around my d. knowing that any second, they could exchange my immense pleasure for world-shattering psychological trauma and probably some severe physical pain is pretty fucking sexy desu. but i am going to have to insist that they keep their eyes closed. i just don't think i can look down and see those big, beaty, black things and be okay with it.