SOLITARY FORAGING IN SAND PITS BY BREEDING CORMORANTS PHALACROCORAX-CARBO-SINENSIS - DOES SPECIALIZED KNOWLEDGE ABOUT FISHING SITES AND FISH BEHAVIOR PAY OFF

Though most Cormorants from the colonies exploiting the lake IJsselmee r area in The Netherlands nowadays have taken up the habit of fishing in large groups, some individuals are still observed fishing solitaril y. This phenomenon occurs predominantly during the coldest months at w ell-defined patches, often located in sand pit regions. A case study i n a sand pit in the early spring of 1982 revealed that Cormorants from Oostvaardersplassen took advantage of the fish located at their winte r refuges inside the pit by specifically diving along the pit's steepe st slopes. Thus, they were able to achieve a more than average daily r ation of about 475 g of fish within a mere 15 minutes of foraging in t he second half of March. Later on, the site became less profitable and the birds gradually left. The importance of high intake rates in earl y spring is stressed, in view of time consuming activities like nest s ite occupation, pair formation and accumulation of body reserves for e gg-laying, incubation and chick feeding. It is suggested that only the 'higher quality' birds may have the experience and skill required to use solitary fishing techniques as a profitable alternative to social fishing in early spring.