Preserving family bonds in birds during seasonal migration is uncommon. Among passerine birds, migration of family groups has not yet been genetically confirmed. Here, we tested whether long-tailed tits migrate in family groups in autumn. We studied two partially migrating populations of the nominate subspecies Aegithalos caudatus caudatus (in Northern Europe and Northeastern Asia). We used microsatellite analysis to check if the individuals within each flock were genetically related. We revealed that in both populations, all flocks contained a large number of relatives. The average proportion of pairs of related individuals within a migrating flock was 73% (from 41 to 100%) in the Northern European population and 61% (from 38 to 100%) in the Northeastern Asian population. Inthe Northern Europeanpopulation, the percentage of pairsofrelatedindividualsinmigratingflockswassignificantly higher compared to Northeastern Asian population. We did not find any evidence that the proportion of the related birds within a