Schlosser, W. & U. Bühler
(* = Kurzbeitrag)
Langjährige Entwicklung des Brutbestands und Brutgeschehens beim Habicht Accipiter gentilis in der Nordostschweiz.
(von 1994 bis 2006 vergeben)
Bestandesentwicklung, Bestandeserfassung, Bestandesdichte, Reviergrösse, Identitätsbestimmung, Mauserfeder, Altersstruktur, Brutperiode, Bruterfolg, Witterung, Störung, Aushorstung, Neststandort, Wechselhorst, Beringung, Abwanderung
Schweiz, Mittelland, Zürich, Thurgau, St. Gallen, Aargau
Long-term trend of breeding numbers and demographic parameters of Northern Goshawk Accipiter gentilis in northeast Switzerland. In an area of 1121 km2 in the surroundings of Winterthur a population of Northern Goshawks was studied intensively over a period of 25 years. The number of breeding pairs remained stable from 1984 to 1999. Thus the preceding period of increase between 1978 and 1983, which had followed a period of low numbers in the 1960s came to an end. The observed density of 1.8–2.7 breeding pairs per 100 km2 was lower than the median value of several European studies. In part of the study area, characterised by a high landscape variety, a density of up to 3.7 breeding pairs per 100 km2 was reached.
Breeding success was determined over an area of 3969 km2. With a mean of 1.52 fledged young per initiated brood, breeding success was relatively low in comparison to other European populations. The number of territories where Goshawks were present but did not breed was higher than the number of brood failures. Breeding success was negatively correlated with the amount of precipitation during the breeding season. In 67 % of all cases of successive broods
in the same territory the same female was present in the second year, which indicates a high survival rate. The percentage of one-year old birds among breeding females was only 0.25 %, which we interpret as a low effect of human activities. None of the 43 recoveries of Goshawks ringed as nestlings indicates dispersal over large distances.
The stability of breeding numbers, the significant percentage of non-breeding birds and the regular spacing of the nesting places in the presence of a sufficient supply of suitable nest sites indicate that the size of the breeding population in the study area is limited by food availability.
A few cases of direct persecution of Goshawks were documented. Forestry activities close to nesting trees rarely led to brood abandonment. With increasing tree-cutting activities during the breeding season, however, the rate of abandoned broods might increase. The problem of chemical pollution should be followed also in the future.
From 1984 to 2003 the breeding season started increasingly earlier, at a rate of 2.9 days per 10 years, measured from the time when the young were first observed outside the nest. During the same period mean temperature in the pre-breeding season increased significantly, and was positively correlated with the date of fledging.
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