SEASONAL NICHES OF NEARCTIC-NEOTROPICAL MIGRATORY BIRDS: IMPLICATIONS FOR THE EVOLUTION OF MIGRATION

THE EVOLUTION OF avian migratory systems has been of intense interest to ornithologists (Gauthreaux 1982; Berthold 1988, 1993; Levey and Stiles 1992; Rappole 1995; Chesser and Levey 1998). The consensus is that a sedentary ancestor began to move seasonally, within an ancestral distribution, in response to some influence-either a push (e.g. poor conditions on the ancestral distribution) or a pull (e.g. better conditions elsewhere) -and that local tracking eventually extended to long-distance, predictable movements (Berthold 1993, Rappole 1995). One particularly well-developed hypothesis is that local movements of tropical bird species tracking variable or uncertain resources (either food or space for breeding) evolved into regular seasonal and longer distance movements. That line of thought, originally based on NearcticNeotropical migrants (Levey and Stiles 1992, Rappole 1995), has been partially supported in an independent system, the austral migrants in South America (Chesser and Levey 1998). New perspectives on the evolution of migration may prove useful (Zink 2002). A question that has not received sufficient attention is the degree to which a species is tracking a single set of conditions year-round-as opposed to changing from one ecological regime to another. Seasonal differences in habitat use by migratory birds are well known (see reviews in Keast and Morton 1980, Hagan and Johnston 1995).