Generally, a male bird is a "cock", and a female bird is a "hen". Male chickens are "roosters" (US) or "cockerels" (UK) or "cocks" (general). You could refer to one as "he" or "it"; but "she" is definitely not suitable (he cannot be a girl!).
(This answer could be made more complicated with regional variations and the using of different words based on the birds age but it is not necessary to go there.)