A Possible Navigation Sense in the Ear of Birds

Abstract

With the reawakening of interest in bird navigation and homing, stimulated chiefly by physicists (Ising, 1946; Yeagley, 1947), it has seemed desirable to re-examine the sensory basis for this extraordinary phenomenon. The present re-consideration of the ear in birds attempts both to interpret structures long known and to evaluate new or little known structures which may cast further light on extra-auditory functions. No formal review of the literature will be made. Many of the earlier references appear in the account of Gadow and Selenka in Bronn's Thier-reichs (1891) and more recent works will be referred to in text. The ear of birds is of course divided into outer, middle and inner ear or labyrinth. Although the latter is also vitally concerned with hearing, our primary object here is to investigate its other functions-first among them, equilibration. The Semicircular Canals.-The modem literature on the semicircular canals of the labryrinth goes back at least irto the 18th century and even the classical physiological experiments of Flourens, firmly establishing the connection of the canals with equilibration, were carried out in the 1820's. Hence it is rather strange that certain clear-cut limitations of this system have not been sufficiently emphasized. There are three semicircular canals, two vertical and one horizontal (fig. 1) and these, being oriented roughly in planes lying at right angles to each other, occupy the three dimensions of space. The bony canals are seen to occupy a position far posterior in the otic region of the skull to either side of the foramen magnum, each of the six enlarging into an ampulla at one end. Within the bony canals (A) and separated from their walls by a fluid, the perilymph, are membranous canals (B) containing yet another fluid, the endolymph. Each membranous canial likewise enlarges terminally into an ampulla containing a crista acutica (C) -a crest of sernsory hair cells innervated by the vestibular branch of the acoustic nerve (see plates in Retzius, 1884). Finally, attached to the crista and enclosing these hair cells is a gelatinous cupula (D) described by Steinhausen (1934), functionally a swinging door mounted transversely inside the ampulla and responding to movements of fluid within the canals. This is obviously an inertia system. If a bird lifts its head upward in the plane of the left anterior canal (fig. 1, upward), the endolymph, remaining behind, gives the effect of flowing forward in the canal toward the ampullathe force and acceleration beinig equal and opposite the movement. In any such head movement of a bird the bill scribes an arc through space, the center of which is a point in the neck usually close to its junction with the skull. Therefore, many of the small, rapid head movements of birds in peering and