Special case among passerine birds: long-tailed tits keep family bonds during migration

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. In the Northern European population, the percentage of pairs of related individuals in migrating flocks was significantly higher compared to Northeastern Asian population. We did not find any evidence that the proportion of the related birds within a flock changed with the progress of the season. We assume that inherent sociality of the long-tailed tit determines the preservation of family bonds during migration. Significance statement Birds generally migrate singly or in flocks consisting of non-related individuals, as young disperse shortly after fledging. Among Passeriformes, migration of family groups has not yet been genetically confirmed. In this study, we used molecular methods to demonstrate that long-tailed tits A . caudatus caudatus keep family bonds during autumn migration—the first verified case among passerine birds. We studied two partially migrating populations in Northern Europe and Northeastern Asia using genetic relatedness analyses and showed that in both populations all flocks contained a large number of relatives. Sedentary subspecies of long-tailed tit are well-known to preserve family bonds, thus, we suggest that family bonds have been conserved in migratory subspecies. These findings may give an insight into the evolution of social behaviour in animals and into the ecology of migration in birds.