Trophic ecology of Yellow-billed Loons (Gavia adamsii) and Common Loons (G. immer) during the non-breeding season

Cover photo: Common Loon Gavia immer with prey. Photo: Torborg Berge. Diet and aspects of feeding behaviour of Yellow-billed Loons Gavia adamsii and Common Loons G. immer were compared during the non-breeding season in coastal Norway. Stomach contents from 14 Common Loons and 13 Yellow-billed Loons showed that fishes constituted the main prey of both species. However, saithe Pollachius virens constituted a major component of the diet of the Yellow-billed Loon, whereas wrasses (Labridae), flatfish (Pleuronectidae), and sandeel Ammodytes tobianus were important prey for Common Loons. Common Loons also frequently included crustaceans and sometimes molluscs in their diet. We tested the hypothesis that the Yellow-billed Loon, with its more upturned bill, feed more on bottom-living prey than the Common Loon. Contrary to the hypothesis, stomach contents indicated that more bottom-living prey were taken by Common Loons, and Yellow-billed Loons seemed to prey more on pelagic fish. A difference in prey selection was consistent with field observations that Yellow-billed Loons often fed in water judged too deep for bottom-feeding, social groups were often synchronized when feeding, and their stomach contents contained only a few gastroliths compared to Common Loons. Species differences in number of gastroliths indicate that Yellow-billed Loons are feeding on softer food such as pelagic fish, whereas Common Loons are feeding on crustaceans and the more armoured and spiny fishes occurring in bottom habitats.