FREQUENCY OF MIGRANTS AND MIGRATORY ACTIVITY ARE GENETICALLY CORRELATED IN A BIRD POPULATION - EVOLUTIONARY IMPLICATIONS

Most migratory bird populations are composed of individuals that migra te and individuals that remain resident, While the role of ecological factors in maintaining this behavioral dimorphism has received much at tention, the importance of genetic constraints on the evolution of avi an migration has not yet been considered. Drawing on the recorded migr atory activities of 775 blackcaps (Sylvia atricapilla) from a partiall y migratory population in southern France, He tested two alternative g enetic models about the relationship between incidence and amount of m igratory activity. The amount of migratory activity could be the conti nuous variable ''underlying'' the phenotypic expression of migratory u rge, or, alternatively, the expression of both traits could be control led by two separate genetic systems, The distributions of migratory ac tivities in five different cohorts and the inheritance pattern derived from selective breeding experiments both indicate that incidence and amount of migratory activity are two aspects of one trait, Thus, all b irds without measurable activity have activity levels at the low end o f a continuous distribution. below the limit of expression or detectio n, The phenotypic dichotomy ''migrant-nonmigrant'' is caused by a thre shold which may not be fixed but influenced both genetically and envir onmentally. This finding has profound implications for the evolution o f migration: the transition from migratoriness to residency should not only be driven by selection favoring resident birds but also by selec tion for lower migratory activity. This potential for selection on two aspects, residency and migration distance, of the same trait may enab le extremely rapid evolutionary changes to occur in migratory behavior .