Airplane Observations of Homing Birds

Abstract

It is common knowledge that many birds successfully find their way over long distances-either during natural migrations or in artificial homing experiments when they return to their nests after being captured and transported hundreds of miles before release. These feats of navigation are often so spectacular (for instance the Pacific golden plover flying 2,000 miles from Alaska to the Hawaiian Islands) that none of the known sensory mechanisms seem adequate to guide the birds' flight. At first glance one is certainly inclined to feel with Chapman ('37) that . . something far more potent than eye, ear and memory is evidently required to lead birds over journeys where landmarks are wanting. Indeed many ornithologists postulate that birds are guided in their flights across whole continents and oceans by an undefined sense of (Mayr, '37; Rfippell, '36). It is largely this possibility of an unknown sensory mechanism which has intrigued the attention of biologists. However, bird migration is also an ecological problem. Obviously the birds are traveling through an environment which is varying in its ecological dimensions, and whatever directional cues guide the birds' flight provide important relationships between them and their environment. But beyond this general consideration there is recent evidence that many birds find their way not by relying on a special sense of or other unique sensory mechanism, but rather by an ability to perceive environmental cues which are within the scope of the receptors common to all higher vertebrates. If this view proves generally correct, the problem will become primarily an ecological one; and since the observations reported in this paper tend in this direction it seems appropriate to present them in ECOLOGY. The belief in an unknown sensory mechanism responsible for bird navigation has been based to a great extent on artificial homing experiments as well as on natural migrations; for the former yield concrete data regarding the speed and direction of specific flights of individual birds, while migrations are ordinarily studied by observing the movements of whole populations. Even the recoveries of banded birds, while they have been of great aid in tracing migration routes, seldom reveal much about the exact course flown or the means of navigation employed. There is almost always a considerable lapse of time between banding at one locality and recovery at some distant point, during which the bird may have traveled widely and strayed far from a straight line between the two. In an earlier review (Griffin, '44) which considered in some detail the results of almost all homing experiments on record, one of us was led to the conclusion that most of the data could be explained by assuming: first, that birds have a well developed topographical memory, so that having flown over an area in migration or natural wanderings they can thereafter orient themselves within that familiar territory by means of landmarks; and second, that when artificially transported to unknown territory they explore wide areas until they are able to see familiar landmarks. This conclusion was supported by the following facts: (1) the homing speeds of wild birds are far below their ordinary velocities of flight, even making reasonable allowances for rest and feeding; (2) the speed of return and the percentage of birds returning are usually greater from familiar territory than from equivalent distances extending into unfamiliar areas; (3) birds which do return from unknown territory and are then