Feeding associations between Red-necked Grebes Podiceps griseigena and Velvet Scoters Melanitta fusca in winter

Red-necked Grebes Podiceps griseigena wintering along the coast of Jaeren, southwest Norway, were found to feed either solitarily or in close association with Velvet Scoters Melanitta fusca feeding on bottom prey (mainly echinoderms) dug out from the sandy substrate. By scuba diving at spots where both species were feeding, we found that stirring the sand caused polychaete worms to emerge from the substrate. Bottom samples showed that these worms (on average >1000 per m(2)) vastly outnumbered other animal taxa living in the substrate and that polychaetes were probably the most likely prey of grebes associated with scoters. Food competition between the two species and/or kleptoparasitism by the grebes were probably not involved (no food-stealing attempts by the grebes toward scoters handling prey brought to the surface, no aggression by scoters toward grebes and no effect of the grebes on the time budget of the scoters). Fed-necked Grebes associating with scoters were mainly first-year birds, and the frequency of association decreased from a maximum of nearly 60% of the grebes in November to c. 5% before spring departure in April. Grebes feeding solitarily hunted in a manner resembling divers (Gavia), and they brought fish to the surface significantly more often than grebes feeding with scoters. Solitary grebes spent less time feeding and more time preening and swimming than did grebes associating with scoters, The association appears to be a way naive birds could easily obtain prey. Skills needed to dive for more nutritious but agile fish are probably gradually acquired through the winter as more of the grebes adopt solitary feeding.