Differential habitat occupancy by goldeneye ducklings (Bucephala clangula) and fish: predator avoidance or competition?

Two hypotheses have been proposed to account for disjunct habitat occupancy between breeding Common Goldeneye (Bucephala clangula) ducks and fish: competition and predator avoidance. Both experimental and correlational evidence suggest that competition for food explains why goldeneyes and their broods tend to use habitats in which numbers of fish are low. However, rejection of the predator avoidance hypothesis has been based on correlational evidence only. In experiments, we tested the hypothesis that avoidance of predatory fish affects habitat use by goldeneyes by examining the responses of goldeneye ducklings (hatched in captivity from eggs collected in the wild) to the presence of predatory and nonpredatory fish models in an aquatic arena. Although response behaviours generally decreased with age, this was not the result of habituation to the testing procedure; responses of the oldest ducklings (9 weeks old) were not different from those of ducklings of similar age who had not been previously exposed to the models. When the presence of the models affected duckling behaviour, group cohesion increased and diving activity decreased, but the intensity of these responses did not differ toward models of predatory and nonpredatory fish. Furthermore, measurements of duckling movements did not indicate that ducklings were actively avoiding the models. Our experimental evidence indicates that avoidance of predatory fish does not appear to be an important determinant of habitat use by goldeneye ducklings.