Tests for species interactions: breeding phenology and habitat use in subarctic ducks

We tested for interactions among five duck species breeding on a set of 236 ponds over 4 yr. Species differed significantly in the time of hatching of broods and in their use of pond sizes. Abundance of broods was significantly correlated with pond perimeter, and hens with broods appeared to be more selective of pond sizes than breeding pairs. Because of the large number of replicates of isolated habitat units, i.e., ponds, we could perform several tests of the hypothesis that competition among broods determines the distribution and abundance of broods in time and space. In the aggregate, our analyses indicate that the degree of overlap among species is proportional to the intensity of competition as estimated by several methods. This result implicates that interspecific competition, past and present, is responsible for the differential use of pond sizes by ducks, the complementary occurrence of certain species on equivalent ponds, and in part, the divergence of early and late life history patterns.