The British slang use of “bird” for a woman has an interesting double history. The modern slang dates back to around 1915 WordReferenceEtymonline, when it emerged in British usage similar to how Americans use “chick.” However, there’s a much older root to the term.
Back in Middle English around 1200, there was a word “burde” or “burd” that meant “maiden, young girl, or woman of noble birth” Etymonline. This older word was possibly a variant of “birth” meaning lineage, and got confused with “bride,” but eventually fell mostly out of use except in alliterative poetry and phrases.
The modern slang meaning likely arose independently of the medieval word, though there may have been some influence. The connection people made was to the qualities they associated with birds—gentle, free-spirited, light, or needing protection Know Animals. These characteristics got tied to old-fashioned ideas about women.
The term really took off in the 1960s and 70s, appearing everywhere in British movies, conversation, and culture Know Animals. You’d hear young men talk about taking “their bird” to the cinema or on dates. It’s still used today, particularly in the north west of England, where phrases like “I’m going round me bird’s house” aren’t especially unusual Quora.
That said, opinions on the term are mixed nowadays. Some people find it friendly or affectionate, while others consider it outdated, infantilizing, or even disrespectful. A 2016 survey showed “bird” topping the list of “pet names” that British women would like banned from everyday use WordReference.