COURTSHIP FEEDING [Ref]
A young man, giving his best girl a box of chocolates, and a bird, giving his prospective mate a worm or a berry, have this in common: they are both practicing courtship feeding. Further, humans and birds are the only vertebrate animals that do this.
With birds, during courtship, the female often begs to be fed by acting like a young bird—with fluttering wings and widely gaping mouth. The male normally places the food he has collected directly in the open mouth of the female.
The significance of this courtship feeding has been discussed especially by David Lack, in the scientific journal, the Auk. It seems that in courtship feeding the food as such is not of primary importance. The female does not need the food she is begging for; indeed she may have had a full meal since her mate, whom she is soliciting, had last eaten. Perhaps it is of help in maintaining the bond between the pair during the period that exists before they have a nest and young to look after. In this connection it is interesting that with waxwings during courtship feeding the fruit that the male gives the female may be "handed" back (by beak) and the food exchanged back and forth.
In looking for significance and correlations in courtship feeding we find that some species practice courtship feeding and some do not. And the birds that do practice it are usually those in which both sexes care for the young. It might be considered an early, useless appearance of a habit that later becomes useful when the male feeds the incubating female and helps feed the young.
This type of behavior, in which an act used elsewhere is introduced into courtship, is sometimes called "symbolic." Other such symbolic acts are the preening that sometimes takes place between a pair of mating birds, and the passing or the manipulation of nesting material long before there is a nest to be built.
Some species during courtship go through actions that resemble courtship feeding except that no food is passed; the bill touching of the mourning dove and of the waxwing falls in this category. Perhaps it is incipient courtship feeding on its way in the long course of evolution, either upward, to include food, or downward, away from courtship feeding.
Their functions seem to be to give something for the pair to do; something they can share. It helps fill up the pair's day and keep them together. It is something that helps strengthen the bond between them, against the day when both will be working together raising a brood.