According to tradition, the birds and the bees is a metaphorical story sometimes told to children in an attempt to explain the mechanics and consequence of sexual intercourse through reference to easily observed natural events such as plant pollination.
Word sleuths William and Mary Morris hint that it may have been inspired by words like these from the poet Samuel Coleridge (1825): 'All nature seems at work ... The bees are stirring--birds are on the wing ... and I the while, the sole unbusy thing, not honey make, nor pair, nor build, nor sing.'
Another use of this phrase comes from the work of John Burroughs, a naturalist who lived and worked in the Catskills Mountains. He wrote a small pamphlet called "Birds and Bees: Essays in which he explained the workings of nature in a way that children could understand. However, birds and bees are dealt with in separate sections; nowhere are they mentioned together.