In the USA, remains of the bird, usually a bloody goo called snarge, are sent to the Smithsonian Institution's Feather Identification Laboratory to determine the species.

and...

Strangely enough, vehicle-animal air collisions also sometimes include species that cannot fly. The Smithsonian Institution's Feather Identification Laboratory has identified frogs, turtles, and snakes as the animal in the "bird" strike. On one occasion they identified a cat at high altitude and on another a rabbit at a height of 550 metres (1800 feet).[1]

The most likely explanation for this incongruity is that the animal identified had been either eaten or carried aloft by a bird of prey, though there are also meteorological explanations, such as raining animals.