The curious keel-like, angular projection found on the palate in the North American Grackles of the genus Quiscalus, recognized as one of the prominent characters distinguishing that group of Blackbirds, is a structure that can hardly fail to attract attention when the mouth is examined in freshly killed specimens, or in birds preserved in spirits. Recently, certain observations made in the field on these birds, which will be recounted later, recalled this structure to mind and the writer was led to make a somewhat detailed study of the palatal keel in the Grackles, and finally to examine the appearance of the palate in other members of the family Icteridæ. In these studies, carried on in the United States National Museum, there have been available suitable specimens representing all of the leading genera with the exception of Clypeicterus, Ocyalus, Lampropsar and Macragelæus. In all, one hundred and thirteen species belonging to thirty-one genera have been examined.
Fig. 1. Head of Quiscalus quiscula æneus.
a. Palatal keel (about natural size.)
Study of skins of the genus Quiscalus shows that the palatal keel is developed as a compressed projection from the roof of the mouth, slightly behind the center of the commissure [(Fig. 1)]. Viewed from the side it is truncated in front, forming an angular projection that has a tendency to become toothed at the tip. Posteriorly it lowers to merge finally into the level of the palate. The anterior margin is sharp, and the posterior portion is thicker and stronger. The entire ridge is developed as a fold in the horny sheathing of the palate, and the surface of the premaxilla underneath is smooth and flat with no indication of a bony ridge to support the keel.
From the examination of museum skins it appears that the palatal ridge begins to develop in juvenile birds a short time before they leave the nest, at a stage when the body is well covered with feathers, and the incoming tail feathers have attained a length of 20 to 25 millimeters. In such birds the keel appears as a very slightly raised ridge that forms a distinct line on the palate. The bill at this time has reached about three-fourths of the length attained when the bird is adult, so that the beginning of this ridge appears to be located far forward, though it occupies the same position in relation to the external nasal opening that the fully developed keel does in the adult. In the dried skins the ridge is somewhat indistinct, but it is possible that it may be more readily apparent in living or recently killed specimens.
In birds that are almost fully feathered and that are about ready to leave the nest the bill has become stronger, the raised palatal line is heavier, and has a rounded anterior end that forms a marked projection and then continues to merge with the palate in front. In older specimens, able to fly but with the rectrices only 95 to 105 mm. long, the palatal ridge was better marked, being broad and strong basally and more slender toward the point. In a few of the specimens of this stage examined the cutting angle seemed well developed, but in others it was less strongly indicated. In birds that were fully grown but still in juvenal plumage the ridge was well developed but not so prominent as in adults. In some the basal portion was broad and rounded, verging toward the formation of palate found in the genus Megaquiscalus. In others the anterior cutting angle was more prominent but the entire ridge had only attained from one-half to three-fourths of its full height.
No one apparently has raised the question of the possible function of this keel, developed as described above, so that it seems proper to record here certain field observations made by the writer that indicate the use of this structure. As might be expected it serves in securing and preparing certain parts of the food. In December 1917, near Stuttgart in eastern Arkansas, during a time when the ground was covered by a light fall of snow, flocks of Bronzed Grackles were found feeding among small groves of a pin oak (Quercus pagodaefolia). The ground under these trees was nearly bare and the birds were working about searching for the small acorns that had fallen and were partly concealed under leaves and low plant growth beneath the oaks. The Grackles were tame and with a pair of binoculars it was an easy matter to watch them at close range. The acorns were picked up, held in the bill and pressed firmly against the keel on the palate, then released, turned slightly by means of mandibles and tongue, and then again gripped strongly. In this way the acorn was rotated until a line had been impressed entirely around the shell. With a little further manipulation the shell dropped off in two halves and the kernel was swallowed entire without further preparation, though frequently it was gulped down only after some effort. After watching one feeding flock for some time I clapped my hands sharply to startle them and then examined the ground where they had been at work. Scattered among the leaves were many acorn shells, most of which had been cut in two in a line transverse to the longitudinal axis. Some had fairly smooth, clean-cut margins, while others were roughened and jagged. In searching through the leaves I picked up one acorn still intact that had been dropped by one of the birds, perhaps when the flock was frightened up, in which a line had been impressed entirely around the center. In this the impressions of the palatal keel were distinctly visible.
When attention was once attracted to this manner of feeding other incidents were noted in which the palatal keel was brought in play. On one occasion on the streets of Washington a Purple Grackle was observed attempting to split open a kernel of corn dropped from some passing dray. The bird held this grain in the slight notch near the center of the bill and pressed it against the angular keel. The grain proved refractory, as it snapped out several times, dropping 8 or 10 inches away, to be seized and again compressed. Watching until it had been dropped I frightened the bird and secured the kernel of corn. On one side four grooves impressed in the hard outer surface were visible showing where, and with what force, the sharp keel had been applied.