Family size dynamics in wintering geese

Many bird populations are made up of social units with differences in size and social status. Of these, the family and flock structure of geese Anserini are among the better known. However, how the association of juvenile geese with their parents in families influences the migration timing and space-use of populations, as well as the events leading to juvenile independence are not well understood. We focus on family size dynamics of the Greater White-fronted Goose Anser a. albifrons on its wintering grounds in the Netherlands and northern Germany, where we gathered 17 years of observation data on foraging flocks and tracked 13 complete families with GPS transmitters. We explored how social status and family size affected wintering site choice and migration timing as well as how and why family sizes decreased. We found that family size decreased strongly during autumn migration, likely from juvenile death due to insufficient fuelling. It further decreased through the winter, here seemingly by juveniles accidentally splitting off during strong disturbance events. Different from previous findings, a large proportion of juveniles became independent during winter. Large families generally arrived later to the wintering grounds, wintered further from the breeding grounds and departed later for spring migration than adults without young. Independent young left for spring migration last. Thus, White-fronted Geese are differential migrants by social status. In combination with the observation of low breeding success in this population in recent years, our findings improve the understanding of its spatial and temporal patterns during winter, and their apparent changes. This can support conservation and management decisions for both White-fronted Geese as well as other large migrants with complex age and social structure.