Opmerkelijke Ruigpootbuizerdinvasie in de winter 2010-2011: oorzaken in een historische en Europese context

Notable invasion of Rough Legged-buzzards in winter 2010-2011: circumstances in an historical and European context
In the winter 2010 - 2011, never before seen numbers of Rough-legged Buzzards Buteo lagopus were observed in Flanders. A strong invasion of this normally scarce over-wintering species of rugged open ground occurred which also did not pass unnoticed elsewhere in Europe. This article thoroughly examines this invasion using documented observations from waarnemingen.be together with other observations from last winter from elsewhere in Western Europe. In order to understand this eruption better, this "Roughleg winter" was also put in an historical context.
The Flemish invasion of 2010 - 2011 was spread out over two clear peaks: a large number of active migrants mid-October and unusual numbers of birds stopping over (long term) from the beginning of December. The October peak occurred just after the highest concentrated migration ever of Rough-legged Buzzards at Falsterbo following a successful breeding season in Scandinavia. The influx in December came after the onset of a very inclement winter throughout Central, Northern and Western Europe. The concurrence of all these factors in one single winter was a key component
in the extent of the range of this invasion.
We suggest, on the basis of our comparisons, in the first instance, that snowfall had an important influence on movements of Rough-legged Buzzards in the winter areas, though not accompanied by extreme cold. It can thus be expected that such invasions will remain unusual in the future if snow-rich winters in Europe, according to present expectations in the literature, in relation to climate change, actually will occur less often and be less severe. The most recent data for recurring cycles in rodent and predator populations in the Far North since the relatively severe winters of 2001, 2007 and 2010, also referred to in this article, suggest that the reduction in winters with long-term snow cover in the 80’s and 90’s was possibly a temporary phenomenon, and further research is required. We suggest there is an important role for amateur observations to map the seasonal distribution of Rough-legged Buzzards in Flanders, as well as in Europe, and for future research into the response of Rough-legged Buzzards to climate change. The Rough-legged Buzzard remains as before a scarce and often difficult to identify winter guest which should be carefully documented, particularly with respect to age and sex. This article closes with an overview of the most important and useful identification
features for the recognition of Rough-legged Buzzards.