HABITAT SELECTION BY BREEDING CURLEWS NUMENIUS-ARQUATA ON MOSAIC FARMLAND

Territory establishment and habitat use by breeding Curlews Numenius arquata were studied during I987 and I988 on mosaic farmland (dominated by dry tillage) at two sites in central Sweden. Curlews preferred to breed in areas with a high proportion of grassland, close to rivers, while dry tillage was avoided. Territories at my study site were larger (mean = 45.2 ha) than in areas consisting entirely of grassland. Territory size seemed to depend on the spatial distribution of grasslands, which suggests that habitat fragmentation forces Curlews to establish larger territories in modem farmland than in areas of grassland. The number of territories in patches of grassland was correlated with patch area, and unoccupied patches were more isolated than occupied patches. However, patch area was a more important factor than isolation, since large patches (> 35 ha) were always occupied. Sown grassland was used significantly more than expected for foraging early in the season, possibly indicating the strong influence of the nutritional requirements in the pre-breeding period on territory establishment. Habitat selection when foraging seemed to be less important late in the season, since there was no significant habitat preference then. During this period distance to the nest site seemed to be more important than habitat, sinced the preferred foraging fields (including fields of all habitats used more than expected by area) were situated closer to nests than the less preferred fields, probably an adaptation to the high nest predation risk. The same fields were mostly preferred in the pre-breeding period also, suggesting that nests were built close to good foraging areas. My results indicate that the decline of the Swedish Curlew population since I950 is caused by changes in land use, resulting in decreased grassland area and increased habitat fragmentation, which probably have affected both breeding and foraging possibilities negatively.