Hen Harriers and Marsh Harriers were conducted in dune and salt-marsh ... ita t
salt marsh dune shrub woodland reed pasture rest. Marsh Harrier (male).
Habitat selection of hunting Hen Harriers (Circus cyaneus) & Marsh Harriers (Circus aeruginosus) on the East Frisian Islands Manuela Schröder1, Nadine Oberdiek1, Jochen Dierschke2, Tobias Feldt1, Julia Stahl1 1
Carl-von-Ossietzky University of Oldenburg, Landscape Ecology Group, 26111 Oldenburg, Germany Gavia EcoResearch, Zedeliusstr. 31, 26384 Wilhelmshaven, Germany contact:
[email protected],
[email protected]
2
Hen Harrier (male) (Circus cyaneus)
Marsh Harrier (male) (Circus aeruginosus)
Introduction 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 the quality and availability of preferred hunting habitats influence 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 breeding pairs of Marsh Harriers are steady or even increasing on the East Frisian Islands. Based on the question whether hunting habitats 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.
Marsh Harrier (female) (Circus aeruginosus)
Source: www.moosbach.at
Hen Harrier (female) (Circus cyaneus)
Photo: J. Bruzinski
Photo: J. Bruzinski
Methods
100%
Norderney Borkum
Source: WATIS
Fig. 1: Location of the study sites: Borkum and Norderney are part of the East Frisian Islands within the German Wadden Sea.
dune 70%
shrub 60%
woodland
50%
40%
reed 30%
pasture 20%
rest
10%
0%
Borkum
Norderney
Fig. 2: Available habitats on Borkum and Norderney, pooling based on TMAP vegetation types (Common Wadden Sea Secretariat 2008).
1,0
1,0
Hen Harrier Marsh Harrier
Jacob's index
Hen Harrier Marsh Harrier 0,5
0,0
-0,5
-1,0
0,5
0,0
-0,5
rest
mudflat
reed
pasture
wood land
dunes
shrubs
rest
mudflat
reed
pasture
wood land
dunes
shrubs
salt marsh
-1,0
salt marsh
Jacob's index
80%
Results and Conclusion
Norderney
Borkum
salt marsh
90%
% available habitat
In May and June 2009 flight activities of Hen Harriers and Marsh Harriers were recorded daily from elevated observation points on Borkum (5 sites) and Norderney (3 sites) for 2h during sunrise, midday and sunset. The following data were collected: - species, age & sex (as far as allocatable) - type of hunting habitat - flight mode (foraging, displaying, soaring, rotatory, distance flight) We distinguished 8 main coastal habitat types (Fig. 2). Habitat preference of foraging male harriers was determined using the Selectivity Index of Jacobs (1974) which accounts for the availability of a habitat type when calculating habitat use.
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 values depict avoidance)
References: Jacobs, J. (1974), Quantitative measurement of food selection, Oecologia, 14, 413-417 Common Wadden Sea secretariat (CWSS) (2008), TMAP handbook-TMAP guidelines for an integrated Wadden Sea monitoring, version 1.0
On Borkum male Hen Harriers show a strong preference for pastures whereas Marsh Harriers preferred to forage in reeds and partly in dunes. On the other hand, on Norderney Hen Harriers preferentially hunted in dunes with shrubs and an otherwise short vegetation canopy. Marsh Harriers only foraged in reed beds (Fig. 3). Both species partly hunted above shrubs, particularly at the edges of their preferred hunting habitats. We detected a distinct difference in foraging habitat selection of both harrier species. Direct interference competition did not play a role and can therefore be excluded as a cause for the decline of Hen Harrier populations on the islands. Other types of competition are more subtle 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 lack of foraging and nesting sites for Hen Harriers in their traditional island habitat in the National Park Wadden Sea.
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).
