Change in Numbers and Nesting Ecology of the Gyrfalcon in the Yamal Peninsula, Russia, from 1981 to 2010.

Abstract

—On the Yamal Peninsula (NW Siberia, Russia), the Gyrfalcon (Falco rusticolus) has been studied for more than 30 years. In this paper we summarize data on the dynamics of its nesting density and ecology during 1981–2010. Nesting density was 0.2–1.6 active nests per 1,000 km2, or 0.3–2.0 active nests per 100 km of river valleys. The density varied between years, with the maximum occurring in the mid-1990s and the minimum in the midto late-2000s. We suggest that the population decrease in the 2000s was caused by a decrease of breeding success and by robbing of Gyrfalcon nests for falconry. By our estimation, the contemporary population on the Yamal Peninsula consists of 15–45 annually breeding pairs. Relatively few cliff sites occur there, and most Gyrfalcons used tree nests (more than 80% of nesting attempts). Average brood size was 2.58 fledglings per active nest (n=76), and higher in rock nests than in tree nests: 3.25 (n=16) and 2.40 (n=60), respectively. Brood size in tree nests decreased from 2.91 fledglings per active nest (n=22) in 1981–1992 to 1.94 in 2005–2010 (n=17). Until the 1990s, Gyrfalcons occupied almost exclusively large tree nests (more than 80 cm in diameter) built by White-tailed Eagles (Haliaeetus albicilla). From the 1990s, they began more frequently to occupy mediumand even small-sized (less than 55 cm) tree nests built by Common Ravens (Corvus corax), Rough-legged Buzzards (Buteo lagopus), and even Hooded Crows (Corvus cornix), though the number of available large tree nests had not decreased. Nest size influences chick survival: small nests are destroyed more often than large nests. Average brood size was 3.0 in large tree nests (n=34), 2.43 in medium-sized nests (n=21), and 1.6 in small nests (n=15). We suggest that the Gyrfalcon’s preference for small nests in recent years may be caused by the consolidation of the snow layer on the nest surface in spring as a result of unstable weather conditions—thaws interchanging with frosts, that may be connected to climate change. Gyrfalcons unable to remove the solidified snow from a large nest are forced to occupy a smaller one. As a provisional measure to reduce chick mortality, we began to install artificial nest-boxes with sheds on trees. Received 29 December 2010, accepted 10 May 2011. STUDIES OF THE BIOLOGY OF THE GYRFALCON (Falco rusticolus) in Russia were scarce until recently (Potapov and Sale 2005, Koskimies 2006). On the southern portion of the Yamal Peninsula, observation of Gyrfalcons began in 1973, when V. V. Kucheruk and colleagues found the first nest of this species on the Shchuch'ya River (Kucheruk et al. 1975). From 1974 to 1985, V. N. Kalyakin annually monitored raptor populations, including Gyrfalcons, and studied their ecology in the basin of this river. He mainly focused attention on diet, food relationships, and role of raptors in tundra ecosystems (Kalyakin and Vinogradov 1981, Kalyakin 1983, 1989). In the same years, data on distribution and breeding of Gyrfalcons on the Khadyta River were collected by zoologists of the Institute of Ecology of the Ural Science Centre of the Science Academy of the Soviet Union (Danilov et al. 1984). Our studies of raptors, including Gyrfalcons, began in the Yamal Peninsula in 1981. We mainly studied their nesting ecology, nesting density, and breeding success. Some results of our Gyrfalcon study in this area have been earlier published (Mechnikova and Kudryavtsev 2008, Mechnikova 2009, Mechnikova et al. 2010). In this paper we summarize trends in the dynamics of the population, nesting ecology, and breeding success of this species from 1981–2010.