Right from the start of this century, field observations and the patient ringing of birds have made available a growing mass of data on the breeding and resting areas of migratory species and on the course, period and duration of their seasonal flights. Considered as a whole, this work on migration morphology commands admiration, and when view ed in detail it reveals fascinating insights into the extraordinary naviga tional performances of many bird species, which find their way over enormous distances. Yet only a few dozen physiologists are actively trying to answer the question of how these...