Black duck – mallard behavioural interactions in relation to hybridization

Black duck (Anas rubripes) numbers have declined over the past several decades in conjunction with a gradual eastward range expansion of the mallard (Anas platyrhynchos). Introgressive hybridization has been proposed as a principal cause of this decline. We studied a sympatric population of black ducks and mallards to determine the relative contribution of forced copulation and mixed pairs to hybrid production and test the common belief that mallards are more aggressive than black ducks. Mixed pairs (14% of all pairs), not interspecific forced copulation, likely accounted for the 2% hybridization rate on the study area. Sexual and hostile aggression was greater among mallards than black ducks prior to the establishment of territories, but thereafter was similar between the two species.