Broedvogels van Noord-Friesland Buitendijks en de invloed van verkweldering op hun aantallen

The population of breeding birds on the salt marshes along the Frisian coast (NFB) has been studied for over more than 20 years. Within this period many factors have changed, affecting both vegetation and bird densities. One of these is the restoration of salt marsh habitat by managed or 'accidental' realignment of dykes. In this paper we evaluate the development of the breeding bird community on NFB and focus on four different parts that were restored to salt marsh in 2001, 1992 and 1990 and 1975 respectively. Since 1991, 60 species of breeding birds have been recorded on NFB, mostly colonial species and meadow birds. Both on the salt marsh area and in the summer polders (separated from the salt marsh by a low dyke and flooded only during exceptionally high tides), the number of breeding bird species has increased. A crash of colonial species (mainly Black-headed Gull Chroicocephalus ridibundus and Pied Avocet Recurvirostra avosetta) after 2000 is assumed to be mainly driven by increased predation (presumably by Red Foxes Vulpes vulpes). Today, highest densities of breeding birds occur in the summer polders. The de-embanked salt marshes generally show declining densities of primary meadow birds, towards densities that are typical of salt marshes. With the restoration into salt marsh, the risk of inundation of nests and drowning of fledglings has increased, reinforced by an increased frequency of storm surges during the breeding season. Also vegetation has changed, to a large extent in interaction with grazing. Although the confounding effects of ditching, predation, local grazing management and regional food availability are relevant when considering the observed trends, a negative effect of restoration on primary meadow birds was nonetheless detected. The strong decrease of colonial species however is not associated with the restoration of salt marsh habitat and has occurred nearly simultaneously in salt marshes along the neighbouring Groningen coast.