One of the characteristics of avian migration is its variability within and among species. Variation in migratory behavior, and in physiological and morphological adaptations to migration, is to a large extent due to genetic differences. Comparative studies suggest that migratory behavior has rapidly and independently evolved in different lineages. One reason behind the high potential for de novo evolution of migratory behavior in sedentary populations seems to be the ubiquity of genetic variation for migratory traits in nonmigratory individuals. In resident lineages, a high degree of hidden genetic variation for migratory traits can be maintained because a migratory threshold determines whether migratory behavior is expressed. Genetic correlations among migratory traits and with other traits of the annual cycle are likely to play a major role in determining the rate and direction of evolutionary change.