The birds of Pasvik / E. I. Khlebosolov, O. A. Makarova, O. A. Khlebosolova [et al. ; English transl. Nikita Chernetsov]. - Ryazan : Golosgubernii, 2007. - 175 с. : ил., портр.

Grouse In our study area the most common grouse species are the Western Capercaillie and Willow Ptarmigan. The Hazel Grouse, Black G rouse and Rock Ptarmigan are much rarer and are practically never recorded during the standardised surveys. The results o f winter transect counts show population cycles in the Western Capercaillie and Willow Ptarmigan with a period o f 4—5 years (Fig. 1.1). During population peaks, Western Capercaillie numbers reach 2 .5 -3 .5 individuals - 10 k m 1. During depressions o f numbers, this figure drops practically to zero. Occurrence o f Willow Ptarmigans varies between 5 .5 -6 .0 individuals • 10 km 1and zero. Population cycles o f both species are synchronised. During the recent 15 years, three cycles have been recorded (Fig. 1.1). Population cycles o f grouse in northern regions are well known, but their reasons remain unclear. They are usually explained by the weather, predation pressure, devel­ opment o f food resources (Semenov-Tian-Shansky 1960). Most probably, population cycles are an adaptation which allows the birds to avoid excessive predation (Khlebosolov 1990). Grouse seem to be able to reach high population densities due to large stores o f vegetative food. This in its turn boosts the numbers o f mammalian and avian predators. Besides, in northern areas grouse occupy relatively open habitats and are exposed to predators. If the numbers o f grouse were constantly high, predation pressure would be very heavy. Population cycles o f grouse do not allow the predators to increase their numbers too strongly and thus limit their impact on grouse populations. Years —■— Capercaillie —*— Willow Ptarmigan Figure 1.1. Dynamics of grouse numbers in Pasvik Nature reserve in 1993-2006. 60