De Kievit Vanellus vanellus in de knel op de ZO-Friese zandgronden

In 1994-99 I counted breeding Lapwings in three study areas in southeastern Friesland. Most plots consisted of arable land. The Fochteloerveld (507 ha) is an open peat-moor reclamation with a dry and sandy soil. Part of this area has recently been transformed into a reserve with semi-natural grassland. The Compagnonslanderijen (108 ha) is a less open peat-moor reclamation and is intensively farmed. The bigger part consists of grassland. The Haulerpolder (119 ha) in a river valley is a relatively wet, mainly open grassland of which a part is managed as a nature reserve. All arable land is intensively used, except for land in nature reserves. Territories were mapped using a standard mapping method (van Dijk 1996). All nests found were marked and checked at least weekly. Daily survival rates of clutches were calculated using the Mayfield method (Beintema 1995). As Lapwings can produce several repeat layings after the loss of earlier clutches, reproduction was calculated as the maximum number of families devided by the total number of pairs. Fledglings were counted in 1997-99. During 1994-99 numbers of territories fluctuated but remained more or less stable (Fig. 1). In the course of this period Lapwings abandoned intensively used grassland and started to breed more regularly on farmland (Fig. 2). Hatching success was highly variable between years and areas (Table 1). However, hatching success was not correlated with the number of pairs producing fledglings (Fig. 3). Different crops resulted in varying hatching successes, largely depending on land management and the attitude of the farmer towards the birds (Table 2). The impact of the Frisian tradition of egg-collecting (allowed until 9 April) was not quantified. However, intensive egg-collecting led to a considerable delay in hatching dates. Whereas local tradition does not allow people to collect eggs on intensively farmed grassland, Lapwings became more vulnerable to the depredations of egg-collectors after they shifted to farmland. The considerable retardation of hatching dates over the years (1- 2 weeks in 5 years; Table 3) can be partly explained by this phenomenon. The effect of delayed hatching dates becomes clear in Table 4. The differences between observed and expected number of young in the study plots increased in the course of the season, implying higher chick mortality. This is probably caused by a decrease in the accessibility of food through growing crops and dehydration of the land. The number of fledglings/pair (0.17-0.34) is far too low to sustain a stable population; the latter requires on average 0.8-1.0 young/pair. It is thought that this local Frisian population is a sink and remained stable through immigration. The only ways to improve reproductive output are a change in grassland management and a ban on egg-collecting.