Aantalsontwikkelingen van overwinterende roofvogels in Nederland in 1980-97

Since 1978, trends of wintering birds in The Netherlands have been monitored by using standardised point transect counts, in which each transect covers 20 fixed points. At each point all birds seen and heard are counted during exactly 5 minutes. The number of transects increased to some 300-400 in 1983/84 and thereafter stabilised; the present transects cover the entire Netherlands (Fig. 1). Trends are calculated with the program TRIM. Hen Harrier Circus cyaneus: the winter index shows a significant decline over 1980-97. Excluding the december 1996 census (with an unusual influx), the Dutch winter trend correlates significantly with the migration figures from Falsterbo (Fig. 3), the main source of wintering Hen Harriers in The Netherlands. The decline was most apparent in dry dunes and on heaths, probably caused by vegetation succession. In farmland, the index remained more or less stable. Goshawk Accipiter gentilis: a significant increase was shown over 1980-97 (Fig. 4). In the core breeding range on sandy soil the population was apparently stable, but the species increased substantially in farmland in the western Netherlands (from <5 to >lO individuals per 100 transects in 1980-97). Sparrowhawk. Accipiter nisus: the winterindex over 1980-97 was stable, unlike the positive trend in breeding numbers. Dutch breeding birds are swamped in winter by birds from NW-Germany, Denmark and Sweden, which obscures the trend in Dutch breeding birds. Common Buzzard Buteo buteo: after fluctuating indices in the 1980s, the index showed a clear increase throughout the 1990s (Fig. 2), especially in the low-lying parts of The Netherlands (where the species colonised hitherto unoccupied breeding habitats). Given stable or slightly declining populations north of The Netherlands, the increase must have been due to an increase of the Dutch population (in accordance with breeding bird censuses). Rough-legged Buzzard Buteo lagopus: although not significant, the trend indicates a decline over 1980-97 (Fig. 6). The species was relatively common in 1981, 1985 and 1995 but very scarce in 1982,1992-94 and 1996-97. Kestrel Falco tinnunculus: the winter trend over 1980-97 is more or less stable, but habitat-related differences were apparent. The decline in dunes and on heaths, as in Hen Harrier, may have been caused by changes in vegetation cover (more grass and shrubs), but the increase in farmland is surprising given the present intensive farming practices. Ups and downs in the trend were not strictly correlated with peaks and lows in vole numbers (Fig. 8). The discrepancy in 1990, a very good vole year and excellent breeding success among vole-eaters like Kestrel and Common Buzzard, can be explained by the fact that vole numbers collapsed in autumn. Merlin Falco columbarius: the indices over 1980-97 were very erratic without a clear trend. It is estimated that some 10% of the NW-European population winters in The Netherlands. Peregrine Falcon Falco peregrinus: the mean number of Peregrines observed per 100 transects showed a distinct increase from 0.9/100 transects in 1980-87 to 2,6/100 transects in 1990-97. It is mainly restricted to the low-lying wetlands and coastline of the northern and western Netherlands. It almost completely disappeared from the central sand district, where prey availability steeply declined (mainly Woodpigeon) and competition with Goshawks and Common Buzzards increased. Presently, point transect counts are the only available method for monitoring wintering raptors in The Netherlands. The coverage of Dutch raptor populations in winter by the present census method varies between 9.6% (Hen Harrier) and 0.7% (both Accipiters)(Table 1).