Foraging and flocking behaviour of migrating and resident frugivorous songbirds

Migrating birds that arrive at unknown stopover sites have to satisfy great er nutritive demands in restricted time and simultanously run a higher pred ation risk than residents which are familiar with the area. Foraging behavi our of resident and migrant frugivores feeding on fruits of Alder Buckthorn (Frangula alnus) was studied in Germany and Spain. German trees ripen thei r fruits during the autumn migration period, Spanish ones during the breedi ng and early postbreeding season. In both areas fruits were mainly eaten by Sylvia warblers, Robin (Erithacus rubecula) and thrushes (Turdus sp.). At the stopover site birds spent less time in the tree, consumed more fruits p er visit and therefore foraged more efficiently. In both areas birds clearl y synchronized foraging bouts and formed intra- and interspecific assemblag es, which were more frequent in migrants than in residents on the interspec ific level. The influence of nutritive demands and predator avoidance upon the foraging behaviour and its consequences for seed dispersal are discusse d.