OUTCOME OF AGGRESSIVE INTERACTIONS BETWEEN AMERICAN BLACK DUCKS AND MALLARDS DURING THE BREEDING-SEASON

American black duck (Anas rubripes) numbers have declined during the p ast several decades, while mallards (A. platyrhynchos) have expanded t heir range eastward. Competitive exclusion of black ducks from wetland s by mallards has been proposed as a principal cause of the decline. W e 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 interspecif ic. When a choice was available, both species interacted more often wi th conspecifics than with the other species (P < 0.028). On wetlands o ccupied simultaneously by both species, numbers of intraspecific inter actions initiated by each species were similar (P = 0.470).!. The prop ortions of won (initiator displaces recipient of attack), lost (initia tor displaced), and ''no change'' outcomes of these interactions were different (P < 0.001). When black ducks initiated interactions with ma llards, black ducks did not lose any interactions and displaced mallar ds 87.2% of the time; no change occurred during 12.8% of the interacti ons. When mallards initiated interactions with black ducks, mallards d isplaced black ducks 63.3% of the time but were displaced by black duc ks 15.0% of the time; no change occurred during 21.7% of the interacti ons. Displacement from wetlands was rare (38 of 229 interspecific inte ractions) and was equal between species.