MAINTENANCE OF POPULATIONS AND CAUSES OF POPULATION-CHANGES OF CURLEWS NUMENIUS-ARQUATA BREEDING ON FARMLAND

The number of breeding curlews Numenius arquata was censused at three large farmland sites (total area 175 km2) in central Sweden from 1985 to 1992. The highest density was found at a mixed farmland site with some traditional farming (mean 0.61 territories/km2) and the lowest density at a modern arable site (mean 0.11 territories/km2). These differences in bird densities were probably effects of differences in the area of grasslands (important foraging and nesting habitats) and wetness between sites throughout most of the period. During the years of study curlew populations increased at the study sites with the largest proportions of grassland throughout most of the study period, while the population decreased at the site with the largest proportion of tillage. Reduced hunting and increased areas of grassland are possible reasons for the increase in population during recent years. The annual adult mortality for adults was estimated to be 17.9 %. Males had a somewhat lower average annual mortality (14.1 %) than females (22.5 %), although the difference was not significant. Adult curlews were philopatric and returned to the same territory in 86 % of the cases. Males (91 % returning to same territory) were more philopatric than females (78 %). Philopatry of adult curlews was shown to be correlated with reproductive success, since a larger proportion of curlews returned if they had hatched a clutch in the previous year. Production of young curlews on farmland dominated by cereal crops was shown to be too low to maintain a stable population. It is therefore possible that birds disperse to modern farmland from bogs, where reproductive success is much higher.