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Aves, 38/4 | 2001 | 179-186
Crossbill invasion 2002 : phenology of active migration during the summer phase.
The premises of a Crossbill irruption were detected at the end of May 2002 and daily migrants counts were performed from June 15th to August 15th at Sart-lez-Spa, eastern Belgium (5°55'E : 50°31'N), to monitor the first phase of the invasion which generally takes place out of the counting period at mogration monitoring stations. The SSW movements peaked during a brief 7 days period at the end of June. They were immediately followed by NNE directed flights that hold until July 12th. This return migration may be due to the lack of Spruce cones in the prospected area or to bad weather occuring in the target zone. In the second case, these bachward movements may look like reverse migration. Adult fully coloured Crossbills dominated during this irruption which, at least partly, seems to belong to the small scale movements of Crossbill populations inhabiting temperate Europe. The frequency of this type of prospection flights increased following the conifers plantations expansion that filled the gap between the boreal and alpine natural areas of the Norway Spruce (Picea abies). During the first part of this Crossbill invasion as well as from the end of August onward, larch (Larix sp.) stands and locally Scots, Corsican and Australian black Pines (Pinus sysvestris, P. nigra var. laricio or var. aistriaca), provide the almost exclusive feeding resources. Observers and ringers attention is called on the lasting presence of Crossbills in some wooded areas where spruce, larch and pines coexist and on the occurence of very small scale mouvements between mast areas.
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