His hair, when it catches the light just so in the dim of the flat, shines with the blue iridescence of a raven's wing, which correlates well with his strangely magpie-like tendencies to hoard shiny objects only he can understand the use of.
Sometimes, when John is unlucky enough to be on the receiving end of Sherlock's sharp, keen gaze, he is uncomfortably reminded of any number of flying predators like hawks and eagles.
One time, when they were on a case, John caught him shouting a repeated "Who? WHO?" at a suspect and so much did the taller man sound like an owl, he had to bite his tongue, it was all he could do not to break down in a fit of giggles. Sherlock may have indeed been incredibly wise in some aspects, but John didn't think he'd find that particular comparison favourable.
When he perches ridiculously on the back of a chair, all long limbs and awkward angles, John is often reminded of some large water fowl – a heron or an ibis perhaps.
And now, tonight, he's having a great sulk, balled up on the sofa surrounded by papers and bits and pieces and oddments, much like some small tree-dweller feathering a nest.
One thing John is certain of – sometimes living with Sherlock really is for the birds.