Speuren naar roofvogelnesten in het Poolse oerbos Białowieża: een lesje in nederigheid

At the end of April and in late July 1999 we tried to locate as many raptor nests and territories as possible in a newly added part of National Park Bialowieza in NE-Poland. The study area (c. 5.000 ha) consists of managed deciduous and coniferous primaeval forest (Fig. 1). Compared to the situation in The Netherlands, where woodland is transected by numerous roads and paths, orientation in the Polish forest was much more difficult owing to wet conditions in many parts of the forest, the sometimes impenetrable sections with dead and dying trees and the absence of paths. Finding nests was further aggravated by the silence of raptors (refusing to give alarm calls when we approached or even climbed the nests) and the near-complete lack of droppings, pellets and moulted feathers near occupied nests. Cold searching for nests in late April, when deciduous trees were on the verge of leafing, was not very successful. We only located 21 nests in five days work by five people. During this period the Lesser Spotted Eagle was the only species of which we collected sufficient data to map its distribution reliably, because its presence was entirely confined to river valleys and glades within the forest (Fig. 1); moreover, late April turned out to be prime time for courtship displays and vocalisations. Nests and territories of Goshawk, Sparrowhawk and Buzzard were incompletely censused, especially because the birds were very silent and nests difficult to locate by cold searching. Observing birds from high tops of Norway spruces at the end of April turned out to be an effective method of locating territories of Buzzard and Lesser Spotted Eagle. During just a few hours of watching, two nests of Buzzards were found rather easily. In July we observed almost the entire study area from the tops of tall Norway spruces (33 hours and 25 minutes in 10 different treetops well-spaced over the region). In this way we traced most territories of Honey Buzzard. Several territories of Lesser Spotted Eagle located in April were confirmed, especially by means of adults transporting prey items to the nests (mice, held in the bill). Because of the poor breeding success, we discovered very few territories of Common Buzzard by means of begging calls of fledglings. Recommendations for more effective location of nests and territories in the future are: – Systematically searching for old nests in November-December, when there is ice but no snow; – Using play-back calls of Goshawks in March and April to find territories; – Observing from treetops from March through July to find territories and nesting sites of all raptor species. The Sparrowhawk will remain a problem, because territories can only be found by systematically searching in potential nesting sites in April and May, a very time-consuming method. The results of our investigation are elaborated in Table 1, and compared with data presented by Pugacewicz (1996). Despite likely differences in methodology, both studies show essentially the same results, except for Honey Buzzard and Lesser Spotted Eagle. The difference in number of Honey Buzzards can be explained by our method of using treetopping, which is very effective to locate territorial pairs. The different numbers of the Lesser Spotted Eagle remain an enigma.