Vijfentwintig jaar stootvogels in het Noordelijk Deltagebied

Since the winter of 1956-1957, the author has counted birds of prey systematically, observed during his excursions in the northern part of the Delta, Southwest Netherlands. In this report the results of 25 years, from 1965 till 1990, have been brought together. Only four species are breeding regularly in the area: Marsh Harrier aeruginosas) Hobby (Falco subbuteo), Kestrel (Falco tinnunculus) and, since 1984, Buzzard (Buteo buleo). The Marsh Harrier increased as a breeding bird from 3-6 pairs in 1970 till 46-57 pairs in 1987. For investigating the numbers of registered migrating and wintering birds of prey, the period from 1st September till 1st April has been taken. The average number of 6 species, observed during the period mentioned in each of the 25 years, has been brought together in two tables. Buzzard and Kestrel increased remarkably after the pesticid period. Hen Harrier (Circus cynaneus) and Sparrow (Accipiler nisus) showed a slight increase. Merlin (Falco columbarius) is rather stable. After an absence from 1964 till 1976, the Marsh Harrier nowadays is a regular winterbird in the area, also in severe winters. During the period of 25 years a lot of changes occurred in the Dutch Delta, owing to the closure of the estuaries. The areas situated outside the dikes, nowadays extensive grassy reservations, appear to be attractive for wintering birds of prey like harriers, Peregrine Falcon (Falco peregrinus) and Rough-legged Buzzard (Buteo lagopus).