"feathered, warm-blooded vertebrate animal of the class Aves," Old English bird, rare collateral form of bridd, originally "young bird, nestling" (the usual Old English for "bird" being fugol, for which see fowl (n.)), which is of uncertain origin with no cognates in any other Germanic language. The suggestion that it is related by umlaut to brood and breed is rejected by OED as "quite inadmissible." Metathesis of -r- and -i- was complete 15c.
Despite its early attestation, bridd is not necessarily the oldest form of bird. It is usually assumed that -ir- from -ri- arose by metathesis, but here, too, the Middle English form may go back to an ancient period. [Liberman]
Still up to c. 1400 it was often used in the specific sense "the young of a bird, fledgling, nestling, chick," and of the young of other animals (bees, fish, snakes) and human children.
The proper designation of the feathered creation is in E. fowl, which in course of time was specially applied to the gallinaceous tribe as the most important kind of bird for domestic use, and it was perhaps this appropriation of the word which led to the adoption of the name of the young animal as the general designation of the race. [Wedgwood]
Figurative sense of "secret source of information" is from 1540s. Meaning "man, fellow, person" is from 1799. Bird-watching attested from 1897. Bird's-eye view "the view as seen from above, as if by a bird in flight," is from 1762. For the birds recorded from 1944, supposedly in allusion to birds eating from droppings of horses and cattle. The bird-spider (1800) of the American tropics is a large sort of tarantula that can capture and kill small birds.
A byrde yn honde ys better than three yn the wode. [c. 1530]
The form with bush is attested by 1630s.