birdyoudaoicibaDictYouDict[bird 词源字典]
bird: [OE] Bird is something of a mystery word. It was not the ordinary Old English word for ‘feathered flying animal’; that was fowl. In Old English, bird meant specifically ‘young bird, nestling’. It did not begin to replace fowl as the general term until the 14th century, and the process took many hundreds of years to complete. Its source is quite unknown; it has no obvious relatives in the Germanic languages, or in any other Indo-European language.

The connotations of its original meaning have led to speculation that it is connected with breed and brood (the usual Old English form was brid, but the r and i subsequently became transposed in a process known as metathesis), but no convincing evidence for this has ever been advanced. As early as 1300, bird was used for ‘girl’, but this was probably owing to confusion with another similar Middle English word, burde, which also meant ‘young woman’.

The usage crops up from time to time in later centuries, clearly as an independent metaphorical application, but there does not really seem to be an unbroken chain of occurrences leading up to the sudden explosion in the use of bird for ‘young woman’ in the 20th century. Of other figurative applications of the word, ‘audience disapproval’ (as in ‘get the bird’) comes from the hissing of geese, and in ‘prison sentence’ bird is short for bird lime, rhyming slang for time.

[bird etymology, bird origin, 英语词源]
bird (n.1)youdaoicibaDictYouDict
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.
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)youdaoicibaDictYouDict
"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)youdaoicibaDictYouDict
"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.