HULSCHER JB & VERHULST S (2003) The rise and fall of the breeding popualtion of Oystercatchers Haematopus ostralegus in Friesland in 1966-2000. LIMOSA 76 (1): 11-22.

Annual mean breeding density of Oystercatchers in 1966-2000 was calculated from counts in a large sample of agricultural sites distributed throughout Friesland and compared with the development of a smaller population on a 28 ha of grassland on the Wadden Sea island Schiermonnikoog. Both populations developed in the same way: rising in 1966-1985 and decreasing in 1989-2000 (Fig. 1). The Frisian population decreased with approximately the same rate throughout the province (Fig. 2). Reproduction was measured indirectly as the mean number of chicks ringed per bird ringer per year (Fig. 3). Population chick production declined (P<0.003), even when numbers of breeding birds were taken into account. The percentage of Frisian-ringed Oystercatcher chicks recovered dead up to five years after ringing significantly declined (Fig. 4). This is interpreted as an indication that progressively more ringed chicks died before they fledged. During the population increase breeding numbers followed reproduction with a delay of about six years and in the decreasing phase with a delay of 12 years (Fig. 3). This suggests that reproduction predominated over mortality in the rising and mortality over reproduction in the decreasing phase of the population development. A simulation of the population size from 1970 onwards, using the indirect measure of chick production (Fig. 3) and data on survival and age at first breeding from a saltmarsh population on Schiermonnikoog, matched the observed pattern reasonably well (Fig. 6). Negative (1983-1986) and positive (1995-2000) deviations from observed values are discussed in relation to possible changes in survival of adult birds in the wintering areas, due to milder temperatures and better feeding conditions. Judging from ringing dates of chicks, Oystercatchers did not change their timing of breeding in 1930-2000, whereas the start of the mowing season progressed with on average 18 days in this period (Figs. 7 & 9). The increased overlap between mowing and breeding is likely to be at least partly responsible for the observed decline in chick production (Figs. 3 & 4). The main conclusion is that reproduction nowadays is insufficient to balance the population (Fig. 5) due to increased losses of eggs and chicks through the intensified use of agricultural land.

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