Reproductie en groei van de jongen in relatie tot de voedselkeus van Buizerds Buteo buteo in het oerbos van Białowieża, Oost-Polen

During the breeding season of2003, Common Buzzards were mapped and studied closely in the primeval forest of Bialowieza National Park in eastern Poland (4000 ha). Leafing of broad-leaved trees was delayed considerably following outbreaks of caterpillars, resulting in a dense herbaceous growth underneath trees. Similarly, high densities of Ips typographus led to massive mortality of Norway spruce Picea abies, also resulting in a denser than normal herbaceous growth in spruce forests. In woodland, both Apodemus flavicollis and Clethrionomys glareolus had crashed after having reached peak numbers in 2002 (source: Patrik Rowyriski). Microtus arvalis and M. oeconomus showed poor numbers and were mostly restricted to a few sites in the vicinity of the village of Bialowieza; the latter species was also largely absent from brook valleys in 2003 (source: Karol Zub). Following a series of dry years, amphibians were far less numerous than in wet years (source: Andrzej Zalewski). Based on long-term breeding bird censuses, bird numbers could be qualified as average, although Turdus merula was more common than usual (source: sampling plots of Universities of Wroclaw and Siedlce). The study area was covered intensively to track down raptor nests, but some may have been missed owing to the dense forest structure. This problem was partly circumvented by posting in the tops of Picea abies overlooking the forest; this tactic was particularly profitable during May (observation time 24 hr) and June (15 hr), but less so in July (65 hr) when few Common Buzzards were active above the forest canopy. All nests showing signs of occupation were climbed at least once in late May, to check for eggs and determine the incubation stage. Around hatching time, nests were climbed again to ascertain precise hatching dates. All nests with chicks were climbed daily, to weigh and measure nestlings, and to collect data on prey remains (species, weights, measurements). Including the climb, nest visits always took 15 minutes or less. By the age of c. 35 days, all surviving nestlings were checked for the presence and number of fault bars in primaries, secondaries and rectrices. No measurements were taken in the last week before fledging (danger of premature fledging). Prey choice was mainly quantified by means of collecting prey or prey remains on nests (n=360); data from pellets were included whenever a prey was identified which was so far missing in the prey remains (n=34). In the Bialowieza National Park, 24 territories were recorded (0.6/100 ha). Eight pairs produced fledglings, of which six nests were actually located (2 other pairs were based on begging calls from recently fledged young and food transportation to an unknown nest site in July). Sixteen pairs did not fledge any young. In seven out of these 16 pairs, a nest was found. None of these pairs appeared to have produced eggs, as deduced from the condition of the nest cup. The distribution of reproducing pairs was restricted to the southern half of the study area (Figure 1), where the vegetation was dominated by Tilia cordata and Carpinus betulus. Reproductive output of Buzzards breeding in the forest edge (399b and 400b, cf. Table 1) was better than of Buzzards in the forest interior, and their start of laying was earlier by 7-19 days. Growth speed (maximum wing chord) was slightly slower and more irregular in Buzzard nestlings in the forest interior (daily growth varying between 70 and 120% of the expected growth, compared to 90-110% in edge-living Buzzards; Fig. 2), and the quality of their remiges was slightly inferior to those of edge-living Buzzards. However, body mass a week before fledging did not differ between the two categories (Table 2). Birds constituted the main prey of Buzzards, i.e. 63% of all prey remains (Appendix 1). Of 246 age-identified birds, 97.6% referred to nestlings or recently fledged young. Twenty-eight birds were found intact, varying in weight between 8.5 g (a thrush nestling) and 70 g (recently fledged starling Sturnus vulgaris), with a mean of 39.4 g. Blackbirds Turdus merula and song thrushes T. philomelos were by far the most commonly captured birds, but all together 21 bird species were identified as prey. Predation on thrushes closely mirrored the peaked timing of fledging in thrushes, as shown by the date at which recently fledged young were found as prey on the nests for the first time: 20 May in song thrush, 2 June in blackbird. After these first kills, fledged thrushes immediately became common prey on the Buzzard nests (Fig. 3). Mammals constituted 21% of all prey remains, of which mole Talpa europaea (12.4%) and root vole Microtus oeconomus (4.6%) were particularly important. Presumably, most moles were dispersing juveniles, as evident from their weights (increasing in the course of June and July) and timing of captures (start mid-June, increasing towards mid-July; Fig. 4). On the other hand, 85% of the root voles were adult, varying in weight between 30 and 45 g (n=7). Amphibians were also captured frequently and constituted 17% of the prey remains. Common toad Bufo bufo was by far the commonest species (78% of 54 identified). All toads were full-grown (44-55 g), but 23% of the frogs was juvenile. Forest-living Buzzards mostly preyed upon birds (65%, mostly thrushes), amphibians (19%) and moles (14%), whereas Buzzards breeding in the forest edge had a higher proportion of small mammals (mice and voles, 44%), besides thrushes (30%) (Table 3). Forest-dwelling Buzzards showed clear individual variations in timing of laying, and consequently in their prey choice during the nestling stage (Table 4). Late- starting Buzzards apparently largely missed the fledgling-peak of thrushes, but probably profited from the increase in above-ground dispersive activities of juvenile moles throughout June and July (Fig. 5). During the egg stage, adult Buzzards behaved low-profile. When approached, most incubating birds left the nest silently (and often unobserved); only a single pair started alarm-calling. A change in behaviour was noticed after hatching, as alarm- calling increased during this stage of the breeding cycle, especially when climbing the nest tree. Most pairs showed learning skills regarding the identification of the observer: alarm-calling was already initiated whenever the observer showed up in the vicinity of Buzzards that were being subjected to daily visits. During much of the nestling period chicks were being attended by the female (Table 5), except at nest 370a where the female was often absent for apparently long periods of time (considering the hungry state of the single chick in the presence of fresh prey items), even when the chick was small. Fledglings were generally very silent; only once during daily visits to the forest in July and August was a begging fledgling heard. Most adult Buzzards with active nests stayed beneath the forest canopy all the time, except for the occasional display flight directed towards a neighbouring pair. During 104 hours of observation from tree tops in June-August, only 4 food transportation flights were recorded in July, covering distances of 0.6, 1.2, 1.2 and 2.2 km respectively. Probably, most hunting took place beneath the canopy, as shown by an adult that changed position every 10 minutes, perching at an average height of 7 m and covering inter-perch distances of 30-50 m for at least half an hour. Although based on small numbers and a single year, and despite various biases, the present study shows that Common Buzzards under primeval conditions have a wide range of options to address temporal and spatial variations in food supply. In this respect, they are not much different from conspecifics inhabiting depleted and humandominated landscapes. These options include: breeding or not breeding (the latter when conditions are adverse), clutch size (small when food in short supply), female nest attendance (prolonged under favourable conditions, or adapted to high predation risk) and variable hunting regimes relative to food peaks within a wide array of prey species. Nevertheless, Buzzards seem to depend heavily on the presence and abundance of voles, alternative food sources being either available for short time periods only (i.e. recently fledged thrushes and dispersing juvenile moles), or less profitable (in terms of nutritional value, such as frogs and toads).