Wow never actually got the meaning:

According to tradition, “the birds and the bees” is a metaphorical story sometimes told to children in an attempt to explain the mechanics and results of sexual intercourse through reference to easily observed natural events. For instance, bees carry and deposit pollen into flowers, a visible and easy-to-explain parallel to fertilization. Female birds laying eggs is a similarly visible and easy-to-explain parallel to ovulation. Another interpretation of the bird laying the egg is childbirth, although that is not as common.

PS: left the links in this time, on iOS usually have to copy to PasteBoard to get plain text - just started happening in Voyager