Rapid 'evolution' of migratory behaviour in the introduced house finch of eastern North America

The house finch (Carpodacus mexicanus) of eastern North America was introduced onto Long Island, New York, around 1940. The source is presumed to be southern California, where ca. 80% of individuals are completely sedentary. The eastern population has become migratory: by the early 1960s, 36% of eastern house finches were performing migratory movements (more than 80 km from their banding site) and that proportion has fluctuated between 28 and 54% in succeeding years. The movements of birds banded during the breeding season and recovered in winter were strongly orientated towards the south-west, and the same pattern was evident in the earliest recoveries (1958-1966); recoveries of birds banded during winter and recovered in the breeding season were orientated toward the north-east. The average distance of migration has continued to increase logarithmically. Areas colonized later, as the range expanded, were characterized by initial long migration distances and high proportions of migrants, suggesting that these traits have evolved in the eastern population. Eastern house finches are partial migrants: not all individuals migrate, and birds that migrate some winters remain in breeding areas in others. Younger birds exhibit a stronger tendency to migrate. A very few western (including southern California) house finches moved long distances, but they did so in directions consistent with seasonal migration, indicating that the machinery subserving migratory behaviour pre-existed in the parent population.