On the Importance of Stopover Sites to Migrating Birds

SOMETHING about the migratory time of year commands our attention.People associate the arrival and departure of birds with imminent change, and the phenomenon generally finds us pondering the various mysteries associated with it all.Given the dramatic nature of migration, it is surprising that relatively little research has been directed toward the study of birds during the migratory period.In part this must be a product of the fact that during migration, individual birds spend relatively little time in any one place compared with other seasons, so when thinking about population regulation and adaptation, we tend to assume that those en-route locations are not all that important.At the same time, however, we have little trouble assigning biological importance to the breeding season, even though it occupies less than a quarter of any year in most birds' lives!Long-distance migrants must travel vast distances, often over inhospitable habitats such as oceans, mountains, and deserts.Flights such as these require that migrants carry large stores of fat for fuel and maintain themselves in top physiological condition.Thus, spring and fall migration are probably important periods in terms of potential mortality.Indeed, Sherry and Holmes (1995) have demonstrated both theoretically and empirically that any time of year, including migratory periods, can act to limit populations.The relative importance of each season to demography simply depends on E-mail: hutto