Pair bonds during the annual cycle of a long-distance migrant,the Arctic Tern(Sterna paradisaea)

Background:The extent to which pairs remain together during the annual cycle is a key question in the behav-ioural ecology of migratory birds.While a few species migrate and winter as family units,for most the extent to which breeding partners associate in the non-breeding season is unknown.The Arctic Tern(Sterna paradisaea)has one of the longest migrations of any species,and the aim of this study was to establish whether or not partners remain together after breeding.Methods:Leg-mounted geolocators were fitted to breeding pairs of Arctic Terns nesting on the Farne Islands,North-umberland,UK.The devices were recovered for analysis the following year.Results:Analysis of data for the six pairs which returned the following year showed that partners departed from the colony at different times after breeding and migrated independently to different Antarctic regions.Partners also departed from the Antarctic and turned to the breeding colony independently.One third of the pairs divorced on return.Conclusions:For long-distance migrants reliant on unpredictable foraging opportunities,it may not be viable to remain as pairs away from the breeding colony.Synchrony in arrival times at the breeding colony may maximise the chance of retaining a familiar partner,but could be affected by environmental factors in wintering areas or along migration routes.