REPRODUCTIVE-PERFORMANCE AND ASSORTATIVE PAIRING IN RELATION TO AGE IN BARNACLE GEESE

1. The pattern of reproductive success (RS) in barnacle geese varied s ignificantly as a function of age. RS continued to increase through th e sixth year and peaked in the eleventh. It declined between the eleve nth and fifteenth year. 2. The decline in RS in old age was not, as su ggested by other workers, due to loss of mate and re-pairing with a yo ung bird; individuals that retained their mates also had diminishing R S 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 a nd feeding area selection, which affects the build-up of body reserves in preparation for nesting and hence breeding potential. Low RS in th e later years was, on the other hand, attributable to the male. We sug gest that this is caused by a decline in fighting ability which determ ines both the acquisition of optimal feeding sites for the female and the ability to acquire and defend a nesting site. 4. At whatever age t hey re-paired, geese tended to choose replacement mates that were simi lar 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.