Reproduction of the Marsh Harrier <i>Circus aeruginosus</i> in recent land reclamations in the Netherlands

We studied temporal variation in reproductive performance of Marsh Harriers Circus aeruginosus in two land reclamations in The Netherlands, i.e. South Flevoland and the Lauwersmeerpolder, embanked in 1968 and 1969 respectively. The number of breeding pairs in Flevoland rapidly increased to a maximum of 350 pairs (± 1 pair km-2) in 1977, followed by a sharp decline in the 1980s due to large-scale cultivation. The same trend was observed in the Lauwersmeer, although colonization was retarded and peak densities were reached later. In both study areas mean clutch size as well as the number of fledglings per nest decreased in the course of the twenty years of study. Two factors were responsible for this decline in reproductive output: (1) decrease of food abundance in the course of the years, and (2) an increase of nest predation, mainly by the Red Fox Vulpes vulpes. Superimposed on these long-term changes, annual fluctuations in density of the Common Vole Microtus arvalis had a considerable effect on the number of breeding pairs, as well as the fledgling production. Mean annual clutch size was associated positively, and laying date negatively, with the average annual temperature during the pre-laying phase. Our results indicate that the early stages of succession, during the first decade after reclamation, are characterized by high prey abundance (i.e. vole 'plagues') and low densities of ground predators, and offer favorable breeding conditions for Marsh Harriers. The simultaneous negative effects in recent years of less breeding habitat, decreased prey abundance and increased predation on reproductive output, exert pressure on these populations. The intense nest predation (50% of all nests), as recorded in the Lauwersmeer since 1990, may eventually cause local extinction in the absence of, relatively safe, inundated breeding habitat