Dynamiek van overwinterende Klapeksters Lanius excubitor
Drentse vogels , Volume 14 - Issue 1 p. 65- 72
In a study area of 45 km5 in western Drenthe (northern Netherlands), the ecology of a wintering population of Great Grey Shrikes was studied from 1990 onwards. The area consists mostly of woodland (twothirds), the rest of farmland (partly laid fallow in the course of the 1990s) and heaths. Wintering Great Grey Shrikes restrict their activities to heaths, sometimes including neighbouring fields. Suitable habitats were checked daily or twice monthly for the presence of shrikes (visual contacts, pellets, prey remains and roosts). Birds were routinely sexed, aged and individually recognised by means of plumage characteristics (cf Schon 1994). The arrival of wintering birds in autumn showed a substantial retardation in the course of the 1990s, concurring with an equally substantial decline in numbers (Figure 1). At the same time, the size of territories in mid-winter increased from an average ofl2.15hain!990(N = 6,SD = 4.47, range = 6.0-16.8 ha) to 89 ha in 1999/2000 (N = 1) (Figure 2). Territory size tended to increase in the course of a winter. One territory in 1990 encompassed 6.0 ha in December-January, 18.1 ha in February and 24.2 ha in March and April. An even greater increase was noticed in a territory in 1999/2000: from 89 ha in mid-winter to 345 ha in late winter (excluding short-term visits to areas 1000-2200 m away). Great Grey Shrikes in the late 1990s and early 2000s are characterised by much greater dispersive activities, presumably caused by profound habitat changes and a shortage of food that can be profitably exploited. In one of the main wintering areas (Wapserveld), conditions were generally excellent for Great Grey Shrikes in the early 1990s, with recently clear-felled woodlots and managed heaths (many prime hunting spots), grazing of cows and sheep (providing dung for dung beetles) and high rabbit numbers (providing a mosaic of closely cropped grassy spots with an abundance of droppings). T en years later, this situation had changed completely. Clear-fellings and open spots on heaths had become overgrown with Betula and grassy/mossy vegetations, grazing with sheep ceased in autumn 1999, cow grazing focussed on abandoned farmland (rather than heaths), the rabbit population crashed (from an estimated late winter population of >200 in 1990 to <15 in 2000- 2001),4outof9 preferred hunting posts disappeared, the water level increased (rendering many spots unsuitable for hunting), the short-cropped area declined from 15.7 ha in,1990 to 1.9 ha in 2001 and dung beetle numbers also crashed (average of 45.3±29.7 tunnels/km transect in autumn 1993 versus 2,0. ±1.3 tunnels/km in autumn 2001). These changes are probably responsible for the erractic, more wide-ranging habits of shrikes from the late 1990s onwards. However, the decline in numbers, started locally in the late 1970s and leading to a nearcomplete disappearance of wintering birds from farmland, has been recorded all over The Netherlands and gained momentum in the 1980s and 1990s. This is thought to relate directly to the substantial decline in breeding numbers in Sweden since the 1970s, following profound changes in forestry practices (Svensson et al. 1999).
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