Do adult and juvenile dunlins Calidris alpina form randomly mixed flocks during fall migration?

Many northerly breeding shorebird species show a separation in timing of adult and juvenile migration. If, in addition to genetic control of migration, learning from experienced conspecifics is advantageous, juveniles should join adult birds during their first fall migration when possible. We here present a method to test if juveniles mix with adults during the period of overlap during southward migration, using dunlin Calidris alpina migrating over southern Sweden as an example. While taking timing differences between age classes into account, we compare flock compositions observed in the field against randomized flock-compositions based on the pool of available individuals derived from the field data. During both the early, the adult-dominated, and later, the juvenile-dominated, part of the season, age classes segregate. Applied to other shorebird species, our method could be used in a comparative sense to evaluate the potential for social learning of migration routes.