The ecology of the black grouse Lyrurus tetrix tetrix and fluctuation of its population density during last 50 years in Gatchina district, Leningrad province

Gatchina District is situated in the center of Leningrad Province and environmental conditions there are typical for the Province. The territory of this district constitutes 2900 km2. 1600 km2 of this territory is covered by forest and 370 km2 - by bogs. Agricultural fields occupy 450 km2. This combination of different types of forests, bogs and agricultural fields is optimal for this species. The highest population density of the black grouse in the entire Leningrad Province overall was observed in 1950-1960, and the distribution of birds was virtually continuous, reaching suburbs of Leningrad (St.-Petersburg). In accordance with the data of the State Game Inspection, the total number of black grouses in Leningrad Province in 1970 constituted about 100 000 birds. It means that in Gatchina District it was 3 300 or higher number, i.e. nearly 3.3 birds per 1 km2 in autumn and winter. After that, a rapid decreasing of the number of these birds began, provoked by the rapid increase of the human activity, including bog irrigation, deforestation, legal and illegal hunting, the use of agricultural chemicals dangerous for birds, etc. As a result, in 1978 the number of black grouses in Leningrad Province decreased by half. The lowest population density of birds was observed in 1980-s in the entire Province, especially in its western parts. As a result, black grouses disappeared from many parts of the Province. At present, the western border of the continuous range of these birds runs along the western border of Gatchina District and across the eastern part of the Luga District. The distribution of the black grouse becomes interrupted far westwards. The decrease in the number of black grouses ceased in late 1980-s, then stabilized, and again began to increase. This process is going slowly, step by step. Nowadays, the population density of the black grouse in Gatchina District determined by authors is 1 bird per 1 km2 in spring and 3-4 birds per 1 km2 in autumn on average. The total number of leks in this District is estimated as 30-35. In the majority of them, 3-7 cocks were observed in April-May, but there are about 5-7 very large leks, where 40-50 cocks displayed simultaneously. Most of leks are located in small rounded forest bogs, but the largest leks are found in big bogs in southwestern and southeastern parts of the District. This system of little bogs in this District is the main reason of the stability of the number of black grouses there at present. The nuptial activity in leks lasts from April till the beginning of May; the incubation of eggs occurs in May; hatching and appearance of nestlings is observed from the end of May till the first 10 days of June. Broods keep together till the end of August - beginning of September. The weight of young males and females at this time constituted 850-1150 g and 650-850 g, respectively. The body size also varies: the length of the wing in young males and females constituted 218-253 mm and 221-239 mm, respectively. Such variability reflect the difference in the age of broods. The moult of young birds to the first adult plumage finishes in the middle of September. In winters, all black grouses are feeding mainly on birches. Birds spend the entire winter in small groups, usually not exceeding 10-12 birds in the flock. Rather recently (only 30-40 years ago) it was possible to meet large flocks uniting one hundred or more birds (males and females together).