The budgerigar, a small species of parrot, can learn new vocalizations throughout life and is therefore widely used as a model system for studying various aspects of vocal learning. It is not known, however, why parrots imitate sounds. To test the hypothesis that vocal imitation in budgerigars is related to pair bonding, we recorded approximately 100 contact calls from each of nine male and nine female adult budgerigars that were unfamiliar with one another and then placed them into pairs. We sampled their contact call repertoire weekly and conducted twice-weekly behavioural observation sessions. We compared contact calls by sonagram cross-correlation and classified them by means of a hierarchical clustering algorithm. This analysis showed that all pairs developed a shared call within an average of 2.1 weeks. Further analysis revealed that eight of the nine male budgerigars imitated the contact calls of their assigned mates, while none of the females imitated the calls of their males. We conclude that contact call imitation in adult budgerigars probably contributes to pair bond formation and maintenance. Prior studies on budgerigars were limited by the lack of a behavioural paradigm to elicit vocal imitation reliably. Our study remedies this and thereby serves as a foundation for future studies on vocal learning in adult animals. Copyright 2000 The Association for the Study of Animal Behaviour.