Why birds and bees?

I’ve long heard about parents having a talk with their kids about “the birds and the bees”. Why specifically birds and bees? I doubt any parent having the discussion with their progeny ever truly mentioned either. If they do like my parents did, they didn’t ever have “the talk” at all. My mother gave me a book. It was uninformative, wasted paper and ink. I’d learned more as a farm boy watching cat…