Partially migratory populations comprise both resident and migratory individuals. Th ese tactics may coexist if their demographic contribution to future generations (i.e. fi tness) are equal or vary temporally with environmental conditions, or if individuals switch between being migrant and resident. Alternatively, the choice of movement tactic can be based on individual attributes such as age, competitive ability or personality. In the latter cases, the two tactics are not expected to have

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... lar average fi tness. In this study, we examined the eff ect of movement tactic on reproductive performance and survival of 82 GPS-marked female moose and their off spring in a partial migratory population in central Norway. Th e results indicated higher growth in the migrating part of the population because migrating females produced more twins than resident females. We found no diff erences in pregnancy rates or survival of adults or their off spring, indicating a net fi tness benefi t of being migrant. We found the average shoulder height of residents to be slightly lower than of migrants, but doubt that this aff ected their migration ability. A more likely explanation is that migratory females are both more fecund and grow bigger because of better conditions in their summer ranges. Th is may be a temporal phenomenon if the fi tness diff erences between migratory and resident moose vary according to environmental fl uctuations.