Habitat Selection of Hunting Hen Harriers (Circus Cyaneus)
Total Page：16
File Type：pdf, Size：1020Kb
Habitat selection of hunting Hen Harriers ( Circus cyaneus ) & Marsh Harriers ( Circus aeruginosus ) on the East Frisian Islands Manuela Schröder 1, Nadine Oberdiek 1, Jochen Dierschke 2, Tobias Feldt 1, Julia Stahl 1 1 Carl-von-Ossietzky University of Oldenburg, Landscape Ecology Group, 26111 Oldenburg, Germany 2 Gavia EcoResearch, Zedeliusstr. 31, 26384 Wilhelmshaven, Germany contact : [email protected], [email protected] Hen Harrier (male) Marsh Harrier (male) (Circus cyaneus) Introduction (Circus aeruginosus) Almost the entire German population of Hen Harriers is breeding on the East Frisian Islands in the Wadden Sea. Reasons for the recent population decline are predominantly unidentified. For reasons yet unknown, habitat characteristics on the islands seem to shift towards a situation unsuitable for successful breeding. We assume that especially Marsh Harrier (female) the quality and availability of preferred hunting habitats influence (Circus aeruginosus) breeding success. The present study is a part of a starting project on the protection of Hen Harriers in coastal habitats. In contrast to the declining population trends in Hen Harriers, numbers of Hen Harrier (female) breeding pairs of Marsh Harriers are steady or even increasing on the (Circus cyaneus) Source: www.moosbach.at East Frisian Islands. Based on the question whether hunting habitats Photo: J. Bruzinski overlap between both species, detailed behavioural studies on foraging Hen Harriers and Marsh Harriers were conducted in dune and salt-marsh habitats on Borkum and Norderney. Photo: J. Bruzinski Methods 100% In May and June 2009 flight activities of Hen Harriers and Marsh 90% salt marsh 80% Harriers were recorded daily from elevated observation points on dune 70% Borkum (5 sites) and Norderney (3 sites) for 2h during sunrise, shrub midday and sunset. 60% The following data were collected: 50% woodland 40% - species, age & sex (as far as allocatable) reed 30% % available % habitat Norderney - type of hunting habitat pasture Borkum 20% - flight mode (foraging, displaying, soaring, rotatory, 10% rest distance flight) 0% We distinguished 8 main coastal habitat types ( Fig. 2 ). Borkum Norderney Habitat preference of foraging male harriers was determined using Fig. 2: Available habitats on Borkum and Source: WATIS the Selectivity Index of Jacobs (1974) which accounts for the Norderney, pooling based on TMAP availability of a habitat type when calculating habitat use. vegetation types (Common Wadden Sea Fig. 1: Location of the study sites: Secretariat 2008). Borkum and Norderney are part of the East Frisian Islands within the German Wadden Sea. Borkum Norderney Results and Conclusion 1,0 1,0 Hen Harrier Hen Harrier Marsh Harrier Marsh Harrier On Borkum male Hen Harriers show a strong preference for pastures 0,5 0,5 whereas Marsh Harriers preferred to forage in reeds and partly in dunes. On the other hand, on Norderney Hen Harriers preferentially hunted in 0,0 0,0 dunes with shrubs and an otherwise short vegetation canopy. Marsh Harriers only foraged in reed beds (Fig. 3) . Both species partly hunted Jacob's Jacob's index -0,5 Jacob's index -0,5 above shrubs, particularly at the edges of their preferred hunting -1,0 -1,0 habitats. rest rest reed reed dunes dunes We detected a distinct difference in foraging habitat selection of both shrubs shrubs mudflat mudflat pasture pasture harrier species. Direct interference competition did not play a role and wood land salt marsh salt woodland salt marsh can therefore be excluded as a cause for the decline of Hen Harrier Fig. 3: Habitat selection of hunting Hen Harriers (male) and hunting Marsh Harriers (male) on Borkum and Norderney in May and June 2009 (positive values depict preference, negative populations on the islands. Other types of competition are more subtle values depict avoidance) and demand further studies. However, habitat change through succession is a likely candidate as driving factor in the population dynamics of both species, resulting in a References: lack of foraging and nesting sites for Hen Harriers in their traditional Jacobs, J. (1974), Quantitative measurement of food selection, Oecologia, 14, 413-417 island habitat in the National Park Wadden Sea. Common Wadden Sea secretariat (CWSS) (2008), TMAP handbook-TMAP guidelines for an integrated Wadden Sea monitoring, version 1.0 Thanks to AG „Landschaftsökologie“ University of Oldenburg, all civil servants of the NLWKN, Jonas Bruzinski (Norderney), Christoph Teuber (Borkum), Martin Reuter, Martin Schultze-Dieckhoff (NLWKN, Norden), National Park Administration ,Niedersächsisches Wattenmeer‘, (Wilhelmshaven)..