What is the X in X-mas?
1776, from Latin passerinus "of a sparrow," from passer "sparrow," possibly of imitative origin. The noun is 1842, from the adjective.
Belonging to the avian order Passeriformes, which includes the perching birds. Passerine birds make up more than half of all living birds. They are of small to medium size, have three toes pointing forward and one pointing back, and are often brightly colored. Larks, swallows, jays, crows, wrens, thrushes, cardinals, finches, sparrows, and blackbirds are all passerine birds.