Real Dictionary

passerine in American English
any such bird
of or pertaining to an order (Passeriformes) of small or medium-sized, chiefly perching songbirds having grasping feet with the first toe directed backward: more than half of all birds are included
adjective: of, belonging, or pertaining to the order Passeriformes, comprising more than half of all birds and typically having the feet adapted for perching
noun: any bird of the order Passeriformes

passerine in British English
adjective: of, relating to, or belonging to the Passeriformes, an order of birds characterized by the perching habit: includes the larks, finches, crows, thrushes, starlings, etc
noun: any bird belonging to the order Passeriformes