A Possible Navigation Sense in the Ear of Birds

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Abstract
This critical review of the avian ear attempts to determine the possibility of detection by it of earth forces useful in navigation. The semicircular canals were found capable of reporting mainly rotational head movements to the brain and the maculae of the utriculus, sacculus and lagena report sustained changes of head position. A new motion sense based on the inertia of air in the tympanic cavity may report thrusting head movements. The rest position of the avian head, different for different species, is one in which the external semicircular canal is horizontal. Since this is the position of the head in flight, even tnough the body may turn and twist in banking, birds may be able to hold the head level even when out of visual contact with the ground. This is an important assumption of the Ising theory of Coriolis force detection and, since the 4 vertical canals are inactive in flight, a streaming of the contained fluid due to this force may actually be detectable when the bird moves its nead from side to side. Their physical characteristics suggest that they could serve as an actual compass. A sac of cavernous tissue, capable of being inflated by venous blood and of covering the ear drum, may cut out air pressures in flight ana further isolate the canals from local barometric variations. Recent evidence of rapid homing in birds suggests that it can hardly be accounted for by random wandering. Fish, eels, turtles and seals, also homing consistently, share with birds locomotion in the fluid media of the earth and the possession of semicircular canals, even though the middle or hearing ear may be absent.

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