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.
THEY TURNED THE TABLES [Ref]
Most birds prey on animals enough weaker than themselves to be in no danger from their prey; their hunting is more like that of the gunner after rabbits than that of the hunter after lions. But there are exceptions.
The great blue heron, armed with a spearheadlike bill, lives largely on fish. These it spears in the water, stalking about after them on its long legs, or waiting like a bird on a Japanese screen, as patient as any fisherman, for its prey to come within striking distance. The heron's size and its great bill render it safe from most enemies. But it sometimes overestimates its ability. Audubon recorded one on the Florida coast that, standing in deep water, up to its belly, struck a fish too large for it. The fish dragged the bird for several yards, now on the surface, now underwater. Finally, after a severe struggle, the heron freed itself. It was exhausted, and stood near the shore, head turned away from the sea. As if, Audubon said, it was afraid to make another attempt at fish catching.
A more serious encounter for the bird was recorded in Field and Stream magazine. The heron had caught a shad about a foot long. He tried to swallow it, but it was too big to go down. He tried to disgorge it, but the fins of the fish, acting as barbs, kept it from slipping backwards and out. The result was death for both bird and fish.
CAUGHT BY A CLAM The oyster catcher, a large black and white relative of the sandpiper, feeds on, among other things, shellfish. Mussels and oysters look like hard nuts to crack, even with a stout, wedgelike bill such as that of the oyster catchers. The oyster catcher's favorite feeding times are when the tide has fallen and the shellfish are first exposed to the air and before they have closed up their shells, and again when the tide is rising and the shells are just beginning to open. The oyster catcher stabs into the shell, and with its bill cuts the strong adductor muscles that hold the shells of the bivalves together. The rest is easy. But a danger lurks here: what about stabbing into too big a shellfish, or making an inept stab? And this very thing has happened. On the South Carolina coast Mr. W. P. Baldwin found a trapped, drowned bird. It was held, with the tip of its bill caught in the shell of a hard-shell clam, as if in a trap. Apparently the rising tide had flooded and drowned the bird.
The raven eats most anything, living or dead, and except for man has little to fear in the northern forests where it lives. Yet from Wisconsin comes a record of one that met his death through a porcupine. The porcupine's quills are a dreadfully prickly covering that one would think would protect it from most encounters. Yet one animal, the fisher, kills and eats it as a matter of course, and wolves and bears sometimes eat them without too many ill-effects from the spines. The slow-moving porcupine has little regard for automobiles, and many are run over on country roads. A porcupine is too big and tough for a raven to kill and the Wisconsin raven had probably fed on a dead porcupine. Stuck through its gizzard was a quill, and another, which had apparently caused its death, was stuck in its heart, having apparently worked there from the digestive tract.