Original Title and Subtitle: Bestandsdichte und Populationsdynamik der Rohrweihe
Circus aeruginosus im nordöstlichen Harzvorland
Population density and population dynamics of Marsh Harrier
Circus aeruginosus in the northeastern Harz Foreland

Summary
In the last ten years (1998-2007) the breeding population of Marsh Harrier Circus aeruginosus was
watched in the northeastern Harz Foreland. Settlement density on a monitoring area of 2000 km1 2
(KF 1) was on average 2.4 breeding pairs (BP)/100 km2. On a smaller and more favourable provided
monitoring area (KF 2 = 253 1cm2 inside KF 1) settlement density was 5.1 BP/100 km2 (1970-1979) and
8.7 BP/100 km2 (1998-2007). In optimal habitats (e.g. “Seeburg” = KF 3) Marsh Harriers are able to
breed colony-like and with very high density (2005: 15 BP/10 ha). Then the distances between two nests
are minimally only 10 m. All nests were in natural habitats (Phragmites, Typha). In the monitoring area
there was only one exception: a second clutch was produced in a neighbouring cornfield.
Strong fluctuations of the breeding population (up to 50 %) depend on population dynamics of small
mammals (common vole gradation) from year to year. Independent of these yearly fluctuations a clear
increase of the population occured on a long-term base. Compared with the 1960s the current breeding
population is about twrice as large.
While the brood sizes (= number of young per successful breeding pair) were on average nearly equal
(3.0 and 3.1) in the KF 2 in both investigated decades, the reproductive success (= number of young per
all closely controlled breeding pairs) was clearly higher (1.5 to 2.5) in the last decade.
In the last two years the Raccoon Procyon lotor appeared as predator. In future brood losses of the
harrier by this predator might increase.