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The Common Buzzard is a partial migrant in Eastern Europe, but its wintering densities are poorly known for this area due to a low number of studies. However, its migration patterns and wintering grounds can be affected by climate change with local fluctuations in numbers linked to weather conditions. Between 2010 and 2015 we annually carried out 15 transects/winter month in order to count the wintering population of Common Buzzards across Eastern Romania to look for climatic factors explaining variation in wintering density. Transects were controlled during two weeks from the end of the month, each covering a distance of 40 km, outlining the broadest winter monitoring of raptors in Central and Eastern Europe. Road survey was used, with a car driving at a low speed, and two observers counting birds, recording distance and habitat where birds occurred. In analyses, in addition to local weather data also 9 indices integrating weather conditions across longer time windows and larger areas were included. The wintering density calculated for the five-year period was 0.539 individuals/km2 fluctuating during the winter seasons with a possible cyclic pattern. The Buzzards selected pastures and herbaceous vegetation associations for hunting across their wintering grounds. Common Buzzard numbers fluctuate during the winter season (highest in December). They are positively influenced by rising solar flux and negatively by the low North Eastern Europe temperatures and Arctic Oscillation Index recorded within 14 days before the survey, which drive birds to southern wintering areas. These weather conditions (high pressure and low temperatures) represent the onset of the Siberian Anticyclone, a regular, annual weather system that occurs in Eastern Europe every winter.

During seven winter seasons (1993/1994–1999/2000) 107 roadside counts (28 km each) in agricultural area of south-western Poland were conducted. From among 1526 Buzzards, 1293 were recorded in winter months (November—February) and 233 in March. Birds were non-randomly distributed on nine vegetation types met along the transect route. Permanent papilionaceous crops, along with cereal stubbles, margin habitats, meadows and maize stubbles were preferentially used, whereas bare tillage and winter cereals were avoided. In general, Buzzards preferred habitats of the least amount in the studied area and avoided those which dominated. These data support the idea of high importance of small landscape structures for the biodiversity protection in intensively used farmland. During winter months, most Buzzards (59.5%) were perched when first sighted, most often on trees and in the middle part of their height. The percent of birds sitting on the ground amounted to 34.7%. The smallest amount of individuals (5....

Studies carried out in the vicinity of Rogow in central Poland showed a 47% rise in the population density of the Common Buzzard, inasmuch as the average number of breeding pairs in the study area of 105 km 2 increased from 18.2 in the years 1982-1991 to 26.7 in the years 2001-2003. Furthermore, an upward trend was observed for each sepa- rately investigated forest complex. Possible mechanisms underpinning this increase in density are discussed.

Both the Buzzard and the Goshawk nested mainly in pines. The mean clutch size in the former was 2.8, in the latter 3.6 eggs per breeding pair. There were statistically significant differences in clutch sizes in the Buzzard in particular breeding seasons. The mean number of hatchlings was 2.3 in the Buzzard and 2.6 in the Goshawk. Brood losses were similar in both raptors — 19% in the Goshawk and 24% in the Buzzard. The breeding success (the ratio of the number of fledglings to the clutch size) in the Buzzard was highest in clutches of 3 and 4 eggs, whereas in the Goshawk a similar level of success was achieved with smaller clutches (2 or 3 eggs). Only in the case of the Buzzard there were significant differences in clutch sizes and numbers of fledglings in the various years. In this species the mean number of fledglings was positively correlated with the rodent availability index in a given year. There was no such relationship between the abundance of prey items found in Goshawk nests and the nu...

In some Buzzard nests with nestlings, located in the Kampinoski National Park, surplus food was found — two or more items of small rodents, mainly voles, without traces of damage, stored in nests. Part of the stored prey was in various stages of decay. Surplus prey items were more frequent in farmland nests than in forest nests.