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The question of how birds find their way during migration and homing has interested the lay public and ornithologists alike. Yet despite all the work that has been done, there are still no definite answers to the question. We know many pieces of the puzzle in the form of cues that birds seem to use, but so far these pieces fail to form a coherent picture. Indeed, there are such large gaps in the puzzle that it seems more than likely that we may not even have all the pieces yet!
During the past 25 years, there has been a great increase in the experimental work on bird orientation and consequently a great proliferation of research papers and reviews. The most useful of these reviews are the ones by Emlen (1975) on migratory birds, Keeton (1974a) on homing in pigeons, Schmidt-Koenig (1979) for a general review on bird orientation and navigation, and Able (1980a) for the general mechanisms of orientation in animals. Given this proliferation of excellent reviews, the only excuse we can offer for yet another is that the field is moving so rapidly that a brief overview of new developments would complement the existing literature and might be helpful to the nonspecialist.
In thinking about bird navigation, there are two major kinds of phenomena we seek to explain. The first is the twice yearly migration of birds, often over distances of thousands of miles.