The Vocal Apparatus of Some North American Owls

Studies of the anatomy and function of the vocal apparatus of owls have not thus far dealt with many of the North American species. Most of our knowledge of strigine syringes is based on Beddard's dissections (1888 and 1898) of scattered representatives of the group, chiefly from the Old World. In examining syringes of eight species of owls from the western United States I have encountered, firstly, some sex differences which are correlated with differences in the utterances of the birds, and, secondly, some facts decidedly at variance with Beddard's descriptions and illustrations. These matters and the functioning of the apparatus appear to merit clarification. I am indebted to Miss Frances Carter, Mr. Lawrence V. Compton, Dr. Loye Miller, Dr. S. B. Benson, and Mr. W. H. Behle for aid in securing materials. The syringeal enlargement of the air passages of owls occurs chiefly in the bronchi where the latter join with the trachea. The enlargement is effected by increased diameter of the cartilaginous, sometimes partly osseous, rings. At the point of enlargementthe rings are much more rigidly bound together than posteriorly along the course of the bronchi. In this way they form on each bronchus a fairly rigid sound chamber well set off from the remainder of the bronchus. The rigidity of structure extends forward through the first few rings of the trachea, but little enlargement is to be noted here in comparison with the remainder of the trachea. The bronchial rings of the syrinx are incomplete dorsomedially, their free ends joining with those of adjacent rings to form well defined margins. Across the space between these margins is stretched a thin membrane like the head of a drum (internal membrana tympaniformis). The meMrbrane is bounded anteriorly by the fusion of cartilages at the junction of the bronchi. Posteriorly it is continuous with a lesser, narrower membrane extending on down the bronchus. The cartilaginous bar that runs dorsoventrally and marks the beginning of the division of the single tracheal passageway into, the bronchial chambers is the pessulus. No discernible vibratile semilunar membrane extends anteriorly from the pessulus, the pessulus merely bearing a sharp anteriorly projecting edge. I have been unable to find any internal muscle fibers in this immediate region. Owls are uniform in the possession of a single pair of intrinsic muscles that run on either side of the vocal apparatus from the lower trachea. to the bronchial ring just posterior to. the solid block of bronchial rings comprising the main syringeal sound chamber. The principal point of variation among members of the group is the length of the bronchial enlargement. Another way of expressing this variation is in terms of the number of the ring to which the intrinsic muscle attaches. In this connection should be mentioned a confusion in the terminology of the rings owing to various views regarding the termination of the trachea. The viewpoint I prefer is to consider the most posterior ring with which the pessulus merges as the last tracheal ring. (The pessulus may merge on the dorsal side only with the penultimate tracheal.) This usage presupposes, in making comparisons, that the location of the pessulus is constant and homologous throughout the group. The other viewpoint is to regard the muscle insertion as a constant point of reference and term the ring involved in the insertion the first bronchial ring. The varying number of rings anterior to it are then termed incomplete or divided tracheal rings-a rather ambiguous designation. By this latter method it is implied that the trachea has split into two tubes