DIFFERENTIAL RESPONSE OF MIGRATORY SUBPOPULATIONS TO WINTER HABITAT LOSS

1. In a previous paper (Goss-Custard et al. 1995), a demographic model suggested that different breeding subpopulations of a migrating shore bird, the oystercatcher Haematopus ostralegus, that shared the same wi ntering area might be affected in different ways by the progressive re duction of their winter habitat. Initially, as winter habitat was remo ved in simulations, all subpopulations decreased in parallel. However, as habitat loss continued, the subpopulations with lower fledgling pr oduction began to be disproportionately affected. This paper explores what caused this difference in response between subpopulations. 2. A s implified version of the model was used in which two or four subpopula tions wintered in one area. Two simple density-dependent functions wer e included, one for breeding territoriality and one for winter mortali ty, with density dependence starting only above a certain threshold de nsity. Each pair that acquired a breeding territory reared a constant number of hedged young each year. 3. As winter habitat was reduced, th e reduction in population size of the subpopulations with lower reprod uctive rates began to accelerate as soon as the point was reached when all adults were breeding. Before this point, breeding birds that had died in the winter had been replaced by non-breeders. After this point , the subpopulation reproductive output was not sufficient to replace birds dying in the winter. At the same time, winter mortality rates re mained high as winter densities were sustained by birds from the other subpopulations. 4. The results suggest that, in order to detect the f irst effects of winter habitat loss, conservationists should pay speci al attention to monitoring breeding numbers in local subpopulations wi th below average rates of reproduction. The results also suggest that the cause of a decline in some local breeding subpopulations, when oth ers remain stable, may need to be sought on the wintering grounds and not in the breeding localities themselves.