During courtship, mated pairs of many bird species touch or clasp each other's bills. Termed billing (also nebbing in British English), this behavior appears to strengthen pair bonding. The amount of contact involved varies among species. Some gently touch only a part of their partner's beak while others clash their beaks vigorously together.
Gannets raise their bills high and repeatedly clatter them, the male puffin nibbles at the female's beak, the male waxwing puts his bill in the female's mouth and ravens hold each other's beaks in a prolonged "kiss". Billing can also be used as a gesture of appeasement or subordination. Subordinate Gray Jay routinely bill more dominant birds, lowering their body and quivering their wings in the manner of a young bird food begging as they do so. A number of parasites, including rhinonyssids and Trichomonas gallinae are known to be transferred between birds during episodes of billing.
Usage of the term has spread beyond avian behavior; "billing and cooing" in reference to human courtship (particularly kissing) has been in use since Shakespeare's time, and derives from the courtship of doves.
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