Food resources and foraging success of Curlews Numenius arquata in different farmland habitats

Abstrakti

The availability of food for breeding Curlews Numenius arquata and their foraging success were studied in different farmland habitats at a study site in central Sweden in the years 1987-1991 . Earthworms were found to be the most important food organisms during the pre-breeding period, when a significantly higher number of earthworms were caught per minute in sown grassland than in tillage. The biomass of earthworms did not differ between these habitats, so the greater availability of earthworms in sown grass was probably due to their intact burrow systems, which made them easier to catch than in tillage, where their burrow systems were destroyed each year during cultivation. Surface-living invertebrates played a minor role as food items during the pre-breeding period, especially in terms of biomass. Significantly fewer earthworms were caught in the breeding period than in the pre-breeding period . In the breeding period the shorterbilled males caught significantly more surface-living invertebrates than the longerbilled females in tillage and meadow, whereas the longer-billed females were able to catch more earthworms than the males. The availability of surface-living invertebrates to Curlews was probably affected by both invertebrate density and vegetation structure. Alarge number of surface-living invertebrates were caughtper minute in tillage, where their biomass was relatively low and the vegetation sparse . In sown grass (with relatively high and dense vegetation), the biomass of invertebrates was high, but the number caught by Curlews per minute low, despite many foraging pecks and probes . The greater availability (not larger biomass) of food items in grasslands and flooded tillage than in dry tillage early in the season is probably a factor responsible for the higher densities ofCurlews in these habitats than in modern farmland, where dry tillage predominates.