Are three age dependent differences in migration patterns and choice of resting sites in dunlin calidris alpina alpina?
Roesner H U.
Journal fuer Ornithologie 131(2): 121-140
The Dunlin Calidris alpina alpina is the most numerous wader species in the whole Wadden Sea. It uses the area mainly for fattening in spring and for moulting in late summer. In the Wadden Sea very strong site fidelity in adults was found. The same is true in the British [UK] estuaries, which are the main winter quarters. This allows the adults to develop a good knowledge of local conditions in the comparatively few areas used. In contrast to the adults, juveniles on autumn migration stop at more places and stay longer at each. They also use sites where only a very few adults are seen. Results of catching and counting in the Wadden Sea suggest that competition between adults and juveniles influences the dispersion of juveniles. To explain these phenomena, a hypothesis on migration patterns is presented. It is suggested that juveniles start their first autumn migration by taking only a rough general direction without aiming to reach particular resting sites. They probably find these by trial and error, visiting only the suitable sites again in the following years. Therefore juvenile Dunlins should discovery newly arisen sites very quickly and may even fill them up. They should not stay at those sites which decrease in quality, so these sites will loose numbrers in spite of the high site fidelity of adults. The hypothesis could also help to explain results which are so far assumed to be caused by the occurrence of different subpopulations.