What is the origin of BrEng ‘bird’ meaning “young woman”?

Bird:

• (Brit.) a girl or young woman, esp one's girlfriend (Collins Dict. )

According to Etymonline, bird:

• "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.

also :

• 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.
  (www.dictionarycentral.com)

Modern usage of bird meaning girl appears to be unrelated to the old Middle Ages meaning. Etymonline dates it back precisely to 1915.

• What is the modern origin of bird meaning young woman?
• In what context did they start to use it at the beginning of the 20th century?

Solution 1:

Oxford dictionary of Word Origins says that the British slang use of bird to mean a young woman is associated with 1960s and 1970s but as you mentioned also, it dates back to Middle Ages. It also adds that the Virgin Mary could be described in those days as "the blissful bird of grace." The modern use appears to be something of a revival.

OED also mentions that this sense of bird was confused with burde , burd n., originally a distinct word and in modern (revived) use, it is often used familiarly or disparagingly. It lists the first usage in 20th century from 1915 as below:

1915 P. MacGill Amateur Army v. 62 There's another bird there—and cawfee!

We might say that the modern usage is revived by media, especially starting from the below usage of News Chronicle article. [It was a British daily newspaper, later absorbed into the Daily Mail.]

1960 News Chron. 16 Feb. 6 Hundreds more geezers were taking their birds to ‘The Hostage’ and ‘Make me an Offer’.

Here, geezer is a British slang for a young lad, bloke (can be an equivalent of dude in AmE). "The Hostage" and "Make me an Offer" were the famous movies of that time.