Population Consequences of Winter Habitat Loss in a Migratory Shorebird. I. Estimating Model Parameters

Abstract

1. In order to construct a model to predict the effect of winter habitat loss on the migratory population of the European subspecies of the oystercatcher, Haematopus ostralegus ostralegus, data on the reproductive and mortality rates collected throughout Europe over the last 60 years are reviewed. Within the Continental and Atlantic regions, inland-breeding and coastal-breeding subpopulations use the same coastal areas in winter. 2. Census and experimental data suggest pairs compete for territories and that an increasing proportion is excluded from breeding altogether, or nest in poor quality habitats, as the number of pairs attempting to breed increases. This provides a main source of density dependence in the basic model. 3. Mean clutch size, hatching success and fledging success were estimated for each subpopulation in each region. Data from one site suggested that the numbers fledged per breeding pair decreases as the total numbers of territories occupied increases, probably because of a reduction in chick survival. This additional source of density dependence was included in some versions of the model. 4. Most post-hedging mortality occurs in winter. Annual mortality was measured from the annual return rates of adults to the breeding areas and probably gives over-estimates. An additional 7-15% of adults die in severe winters once in 7 years in the Continental region but not in the milder Atlantic region. Oystercatchers in their first and second winter have a 20% higher winter mortality rate than adults. 5. The sometimes quite substantial annual fluctuations in the main production and mortality parameters were generally not correlated across sites within a subpopulation. This allowed the standard deviations of the annual variations in these parameters to be estimated for both subpopulations in each region so that realistic annual variations could also be included in the model.