Kan man genom kartstudier förutse häckningsframgång för ängshök (Circus pygargus) i Uppland?

Abstract: Montagu´s harrier (Circus pygargus) is a diurnal, Eurasian raptor, which migrates from its northern breeding grounds to winter in sub-Saharan Africa. In Sweden, its core breeding area is centred on the island of Öland, with a stable breeding population of approximately 40 pairs. The province of Uppland is on the northern end the distribution range, where 4-8 pairs have regularly bred since the 1980s. In Sweden, the Montagu's harrier is classified as an endangered (EN) species. This classification is partly due to a lack of preferred nesting habitat, such as shrub-lands or dense wetlands with Claudium spp. Such wetlands are often drained, resulting in encroachment of bushes and trees into the marshes. In such cases, Harriers usually abandon such areas, and as a consequence >90% of the European population now breeds in farmland. However, breeding in farmland may be problematic for several reasons; mowing or harvesting often results in the death of the chicks and predation from terrestrial predators like foxes. As a consequence of the EN classification, a national action program was recently published describing how to best preserve a viable Swedish harrier population; however, the focus of this program was mainly on the Öland population. Uppland lacks the type of shrub-lands and Claudium-marshes that dominate breeding sites on Öland, thus the national action plan may not be applicable for the Uppland population. In this study I investigate the relationship between the area of open land (hunting grounds and nesting sites) in known nesting areas in Uppland and harrier breeding success. A comparison between nesting sites with high as compared with low breeding success (as defined by the numbers of chicks fledged), did not reveal any significant differences in area of available hunting grounds or area with possible nesting habitat; thus, I cannot find any support for the hypothesis that the quantity of hunting grounds or quantity of possible nesting habitat in the area around the nesting site is important for breeding success for the Montagu's harriers in Uppland.