SWENNEN C & BRUIJN LLM DE (1980) The density of breeding territories of the Oystercatcher Haematopus ostralegus on Vlieland.. LIMOSA 53 (3): 85-90.

The distribution of the breeding territories of the Oystercatcher Haematopus ostralegus was studied in 1977 on the eastern half of the Isle of Vlieland. Most of the isle consists of sand dunes with a very poor vegetation. Locally there are forestry plantations (table I, figure IA). On the Island the Oystercatcher hardly find any food within their breeding territories. Therefore they are forced to search for food in the tidal zone during low tide. Some of the birds feed on the sandy beach of the North Sea. By far most of them, however, go to the tidal flats in the Wadden Sea (figure I B).
      In May 1977,897 breeding territories were identified (8.6 pairs/10 ha). Similar densities have been reported for other Frisian isles. Breeding densities inland are much lower: 0.1-0.6 pairs/10 ha. An important difference is that the inland breeding-birds find their food within their breeding territories.
      The distribution of the breeding pairs in our research area was not homogeneous. This could be related partly to local differences in the study area. In a strip of 75 meter width on both sides of roads and cycle paths densities were relatively low, viz. 1.8 pairs/IO ha as compared to 8.8 pairs/10 ha in the next strip of the same width. Near forestry plantations open to the public, the density was relatively low too. In a strip with a with of 100 meter around these woods a density of 2.8 pairs/10 ha was recorded versus a density of 9.3 pairs/ 10 ha in the next strip of the same width. These lower densities are probably caused by disturbing human activities as such differences were absent around woods where the public had no access. In areas where the density of Herring Gulls Larus argentatus was very high the density of Oystercatcher territories was also relatively low.
      These local peculiarities hardly cloud the overall picture of a consistent increase of breeding density from east to west, from less than 3 in the east to more than 10 pairs/10 ha in{he west (table 2). In addition to this trend also a north-south trend was observed with 4.0 pairs/10 ha in the north, 9.3 pairs/10 ha in the ~entre and 18.9 pairs/10 ha in the south (table 3). These trends show a strong connection with the distance to the tidal zone and its quality as a food resource. On the northern side a relatively narrow sand beach with a low biomass of small prey species is present. Along the southern side mudflat with a higher biomass and larger prey species IS present. These mudflats become larger to the west (figure I B).
      The mean number of growing chicks per pair in 1977 90 was low (0.3/pair) over the whole. area. This observation suggests an ideal free distribution of the breeding pairs (Fretwell & Lucas 1969). In addition to the breeding birds, five groups with about 850 birds in total were found in the area. These birds were not holding a territory. In the central part of the breeding area these birds were mainly adult, on the outskirt of the breeding area they were mainly subadult and on a sand bank outside the breeding area the birds were mainly one year old (table 4). Eurasian Oystercatcher Haematopus ostralegus

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