The distribution and numbers of the Balearic Shearwater during the November-February period are discussed. Until the 1980's this subspecies seemed to be concentrated along the coast south of Valencia. Since 1991-1992 this pattern has changed, and up to 10465 birds have been recorded northwards, along the Catalan coast. The subspecies is distributed along the Valencian and Catalan coasts in nine main areas and numbers vary, reaching a total of ca. 8000-11000 birds. The accuracy of these figures and subspecies identification is discussed. An increase in the wintering population of Balearic shearwaters in Catalan waters has been noted. No parallel increase in breeding populations has been reported. Therefore, a northwards displacement of its main wintering area is a possible explanation. A trophic hypothesis is suggested to explain this winter distribution, given that some important changes in clupeid stocks of the extreme western Mediterranean have been noted. Observed numbers match the known breeding population and post-breeding counts, and thus almost the whole population of the subspecies may occur during the winter in the study area.
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