The Rough-legged Buzzard Buteo lagopus is a Holarctic species that breeds in the Arctic and subarctic regions and winters in a broad range from 40° to 55°N. In Japan, this species is typically a rare winter visitor, but in January 2008 there was an unprecedented influx and 350-400 individuals were observed in western and central Japan. Winter irruptions of this species are considered to be caused by low temperatures and increased snow cover. Here, we describe the distribution of snow cover in mainland Asia leading up to the irruption into Japan. We predicted that movement patterns during spring migration would also be influenced by snow cover. We satellite-tracked four Rough-legged Buzzards from Japan, to show their spring migration routes, and examined the relationship between their spring movement patterns and changes in snow cover along their route. There were two spring migration routes from Japan, which varied with wintering sites. One route led up to the Amur River basin bordering China and Russia. The other extended northwards, reaching the lowlands of the western Chukotka Autonomous Okrug in North-eastern Russia. The bird following the latter route was tracked for two consecutive years and was found using the same summering site in 2009. Shortly before the irruptive movements into Japan, most of north-eastern China and eastern Russia were covered with snow, suggesting that unusually severe weather conditions caused the irruptive movements. The northward movements of one bird tracked to Russia generally matched the northward progression of snowmelt.
2017 The Ornithological Society of Japan