Reproductive Performance and Assortative Pairing in Relation to Age in Barnacle Geese

The pattern of reproductive success (RS) in barnacle geese varied significantly as a function of age. RS continued to increase through the sixth year and peaked in the eleventh. It declined between the eleventh and fifteenth year. 2. The decline in RS in old age was not, as suggested by other workers, due to loss of mate and re-pairing with a young bird; individuals that retained their mates also had diminishing RS in the later years. 3. Low RS in the early years was attributable to the female. We suggest that this is because of inexperience in food and feeding area selection, which affects the build-up of body reserves in preparation for nesting and hence breeding potential. Low RS in the later years was, on the other hand, attributable to the male. We suggest that this is caused by a decline in fighting ability which determines both the acquisition of optimal feeding sites for the female and the ability to acquire and defend a nesting site. 4. At whatever age they re-paired, geese tended to choose replacement mates that were similar in age to themselves. This was despite the fact that, at all times, birds from the young, unpaired cohort (1-2 years) were considerably more numerous than older unpaired geese. 5. We argue that pairing with a familiar individual is advantageous both from the point of view of partner compatibility and of familiarity with the feeding and nesting habitat