Food selection, food-seeking patterns and hunting success of captive Goosanders Mergus merganser and Red-breasted Mergansers M. serrator in relation to the behaviour of their prey

Food selection and food-seeking behaviour of hand-raised Goosanders Mergus merganser and Red-breasted Mergansers M. serrator were tested in experimental situations. When different combinations of two species of fish were presented simultaneously to hungry birds in a small trough where the prey was easily and equally available, neither duck species showed any preferences in prey selection. However, when these same fish species were presented to satiated birds, they showed definite preferences; the most preferred species were baltic salmon and brown trout followed in decreasing order by minnow, whitefish, sculpin, burbot and river lamprey. This was also the case when combinations of five to seven species were presented. However, under semi-natural conditions in a stream tank, the number of each species offish caught was related to their escape behaviour, not to the previously determined predator preferences. When salmon and minnows of different size were presented to hungry birds, they selected the larger fish regardless of species. In contrast, when satiated, they preferred small fish over large fish. Under the experimental conditions, the use of certain elements of foraging behaviour clearly differed between the two bird species: Mergus serrator spent more time hunting underwater than did M. merganser. Results are discussed in relation to the predator-prey situation existing under natural conditions in northern Swedish rivers.