Adoption of young and intraspecific nest parasitism in Barnacle Geese

Prior to use of genetic techniques, extra-pair copulations and intraspecific brood parasitism were rarely observed in long-term monogamous geese. DNA fingerprinting analysis of nine families of Barnacle Geese (Branta leucopsis) revealed one case of intraspecific nest parasitism with the offspring fathered by the attendant male, and one adoption of a foreign gosling. Observations showed that adoptions accounted for 5.8% and 24.6% of goslings hatched, or 13.3% and 24% of families in two successive years. Adoption appears to be common shortly after the young have hatched and has been assumed to result from accidental brood mixing when parent-offspring recognition is not yet fully developed. We found adoptions to occur in goslings as old as 4-12 weeks, when both parents and offspring are capable of recognizing each other, suggesting that accidental mixing alone cannot explain this phenomenon.