Why do British people call women "birds"?

Answer 1

I'll tell you, when you tell me why Americans called the Vietcong "Charlie", deal?

Answer 2

Because the Viet Cong, abbreviated as V C, were referred to by military communications as "Victor Charlie" Just like Able, Baker, Charlie, Delta etc. was used to prevent mistaken communications when referring to other things. The troops in the field created the abbreviated it as "Charlie".
As an Veteran of that war, it was an accept and unmistakable reference. Now, why call British women "Birds"?