PREDICTING THE EFFECTS OF AGRICULTURAL CHANGES IN CENTRAL SPANISH CROPLANDS ON SEED-EATING OVERWINTERING BIRDS

The application of the European Common Agricultural Policy is causing traditional crop cultivations to be abandoned over large areas of cent ral Spain. This study tries to assess the effects of these changes on overwintering seed-eating birds by examining how changes in land use c ould affect winter seed abundances and vegetation structure. Bird dens ities were measured in the winters of 1985 and 1989, and seed densitie s and vegetation structure were measured in 1989, in five major habita t types (grasslands, old fields, growing crops, stubble and ploughed f ields). The food requirements of birds and the abundances of seeds wer e transformed to a common energy currency (kJ 10 ha-1) to allow direct comparisons. Estimated winter food requirements were on average an or der of magnitude smaller than seed abundances across the five habitat types, and the between-habitat distribution patterns of seed abundance and food requirements of birds did not match at all. Bird abundances tended to be inversely related to herb biomass and shrub cover, which were much larger in uncultivated habitats. From the results obtained, we would expect a decrease of the overwintering seed-eating bird popul ations in the area if the decrease in cereal crop is maintained. This decrease appears to be due to the increasingly constraining role of ve getation structure (which probably affects food accessibility and perc eived predation risk) that would outweigh the larger seed abundance of fered by uncultivated habitats relative to croplands.