Modelling the population consequences of age- and sex-related differences in winter mortality in the oystercatcher, Haematopus ostralegus

A modelling approach is used to explore the effect of age and sex differenc es in oystercatcher (Haematopus ostralegus) winter mortality on population size, population structure and the population response to habitat loss or c hange. Increasing the mortality of first and second year birds reduced popu lation size, ut had very little effect on the proportion of the population that were adults. Increasing female mortality reduced population size and r esulted in a male-biased population. A sex bias amongst birds of breeding a ge meant that there were fewer potential breeding pairs for a given populat ion size, reducing the size of the breeding population and the breeding out put. Increasing the mortality of one sex relative to the other reduced popu lation size, even when mean adult mortality rates remained unchanged. Incre asing the strength of density-dependent mortality in young birds caused a g reater reduction in population size as habitat was lost. Increasing the str ength of female density-de pendent mortality had the same effect, even thou gh male density-dependent mortality had been correspondingly reduced, Incre asing density-independent or density-dependent winter mortality in one sex relative to another also exaggerated the disproportional effect of winter h abitat loss on separate breeding subpopulations using the same overwinterin g area, These results suggest that any study of population dynamics should be aware of both age and sex differences in mortality. Conservationists sho uld be particularly aware of any age or sex differences in diet or habitat use that may result in a differential response to environmental change.