What is the origin of the idiom "bill and coo" and when to use it?

The idiom "bill and coo" originates from the courtship behavior of birds, particularly doves and pigeons. In the bird kingdom, "billing and cooing" refers to the affectionate and intimate actions displayed by mating pairs. It involves gentle touching of beaks and making soft cooing sounds as a form of communication and bonding. The idiom "bill and coo" is used to describe the behavior of a couple who are deeply in love and openly expressing their affection towards each other.