Outcome of aggressive interactions between American black ducks and mallards during the breeding season

Black duck (Anas rubripes) numbers have declined during the past several decades, while mallards (Anas platyrhynchos) have expanded their range eastward. Competitive exclusion of black ducks from wetlands by mallards has been proposed as a principal cause of the decline. We studied a sympatric population of black ducks and mallards in Maine during the early breeding season to document behavior and interactions. We observed 832 aggressive interactions; most (72%) were between members of the same species. When a choice was available, both species interacted more often with conspecifics than with the other species (P < 0.028). On wetlands that both species occupied simultaneously, numbers of interspecific interactions initiated by each species were similar (P = 0.47). The proportion of won (initiator displaces recipient of attack), lost (initiator displaced), and ?no change? outcomes of these interactions were different (P < 0.0001). Black ducks displaced mallards during 87.2%, lost none, and no change occurred during 12.8% of the interactions they initiated with mallards. Mallards displaced black ducks during 63.3%, were displaced by the black duck during 15%, and no change occurred during 21.7% of the interactions they initiated with black ducks. Displacement from wetlands was rare (38 of 229 interspecific interactions) and was equal between species. Mallards were neither more aggressive than nor behaviorally superior to black ducks.