Garden Warbler (Sylvia borin) Science Article 2
During spring and autumn migration 2003, the Swiss Ornithological Institute set up a concerted project in Mauritania to study bird migration across the Sahara. I participated with a side project using artificial induction of landfall with sound luring in an attempt to overcome the potential problem of low sample sizes for random trapping of natural landfall. A comparison with sound lured samples was also expected to provide more insights into the motivation of birds to stop or fly. In total, 9.467 migrants of 55 species were ringed, 590 birds recaptured locally and 49 ringed in Europe were recovered. Sound luring was more effective in autumn, when samples became, however, heavily dominated by Reed Warblers (Acrocephalus scirpaceus) and Garden Warblers (Sylvia borin). In autumn, adults migrated ahead of juveniles, and in Reed and Garden Warblers, most adults travelled over the interior of the Sahara, while juveniles were concentrated along the coast; no such effect was seen in species that avoided the interior (Nightingale Luscinia megarhynchos, Grasshopper Warbler Locustella n via). Local recapture rates were low (6%), particularly inland in autumn (1%), where many more birds (12%) made longer stopovers in spring. In autumn, there were many more birds in poor condition along the coast than inland; birds in poor condition had much higher recapture rates, and birds in good condition (fat score >3, muscle score=3) rarely made a stopover. Grounded birds continued migration as fast as possible, many did so in poor condition. Hence a stopover in the Sahara in autumn was primarily a second chance for dropouts unable to continue. More birds in good condition made longer stopovers in spring. Fat was more quickly restored than flight muscles during stopover, eventually resulting in unusual combinations of good fat stores and poor flight muscles. New arrivals of such birds pointed to birds that experienced a near-fatal dropout earlier on; there were more such birds in spring and along the coast in autumn.
Marc Herremans, Report of field research in Mauritania