bird (n.1) Look up bird at Dictionary.com
Old English bird, rare collateral form of bridd, originally "young bird, nestling" (the usual Old English for "bird" being fugol), 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.
Middle English, in which bird referred to various young animals and even human beings, may have preserved the original meaning of this word. 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]
Figurative sense of "secret source of information" is from 1540s. Bird dog (n.) attested from 1832, a gun dog used in hunting game birds; hence the verb (1941) meaning "to follow closely." Bird-watching attested from 1897. Bird's-eye view is from 1762. For the birds recorded from 1944, supposedly in allusion to birds eating from droppings of horses and cattle.
A byrde yn honde ys better than three yn the wode. [c.1530]
bird (n.3) Look up bird at Dictionary.com
"middle finger held up in a rude gesture," slang derived from 1860s expression give the big bird "to hiss someone like a goose," kept alive in vaudeville slang with sense of "to greet someone with boos, hisses, and catcalls" (1922), transferred 1960s to the "up yours" hand gesture (the rigid finger representing the hypothetical object to be inserted) on notion of defiance and contempt. Gesture itself seems to be much older (the human anatomy section of a 12c. Latin bestiary in Cambridge describes the middle finger as that "by means of which the pursuit of dishonour is indicated").
bird (n.2) Look up bird at Dictionary.com
"maiden, young girl," c.1300, confused with burd (q.v.), but felt by later writers as a figurative use of bird (n.1). Modern slang meaning "young woman" is from 1915, and probably arose independently of the older word.