Studies of the orientation behavior of migratory birds and other kinds of animals rely heavily on orientation cages as the means of assaying the animal's choice of direction. In the case of birds, these are usually circular cages in which the direction of the animal's hopping or fluttering is recorded. The integrity of the results of such studies obviously depends upon the researcher's ability at least to know, and often to control, the stimuli to which the birds are exposed. Because the perceptual world of other species often differs markedly from our own, researchers must be extremely careful to be certain that their experimental subjects are responding to the stimuli of interest. Migratory birds have been shown to use patterns of polarized skylight for orientation. This is a visual stimulus to which human beings are barely sensitive, and the physiological mechanism by which vertebrates perceive polarized light is unknown. In addition, it is technically impossible to simulate precisely the polarized-light pattern of the clear sky. Dr. Able's experiments to date suggest that polarized skylight may be centrally involved in the development and performance of orientation in several species of migratory birds. These facts make it extremely important to examine in detail the birds' response to polarized skylight: To what specific aspects of the stimulus are they responding, and how do they use the information as a compass? Knowing these things will help experimenters produce an optimal testing stimulus and apparatus, and will ensure that the birds' orientation is not an artifact based on some unknown aspect of the testing situation.
