Do North American Migrants Follow the Same Route North as South?

The “Migration Route” hypothesis proposes that each migrant possesses a genetically-fixed program providing instructions for the directions the bird must follow and the period of time that it must follow them to arrive at the proper place, i.e., the wintering area in fall and the breeding area in spring, thus predicting that the migrant will pursue the same route north in reverse as that followed south, and that all members of the same species will pursue the same route. We tested this hypothesis for 62 species of migrants captured as transients at a study site located in Texas near the western coast of the Gulf of Mexico during two fall and two spring seasons from August 1973 - May 1975. Numbers captured in spring for 28 of these species were double or more of the number captured in fall and an order of magnitude or more greater for 16, demonstrating that a major portion of their populations do not follow the same route north as that followed south. In addition, the majority of juveniles of some species, e.g., the Yellow-bellied Flycatcher (Empidonax flaviventris), do not follow the same route south as adults. We discuss these data in light of observational information from eBird, banding studies, laboratory experiments, and new technologies, e.g., data loggers and radio transmitters, and conclude that the Migration Route hypothesis is incorrect. The route taken on migration is based on individual choice rather than a fixed genetic program. Each bird uses inherited knowledge of the location of the breeding and wintering areas for its species to follow a path in moving between them (homing) that best fits its own needs based on age, sex, experience, and its immediate physical environment.