Brood sizes of sympatric American black ducks and mallards in Maine

The long-term decline of the American black duck (Anas rubripes) population has been attributed to lower productivity of black ducks that might have been excluded from fertile agricultural wetlands by mallards (Anas platyrhynchos). We monitored broods on 53 wetlands in 1993 and on 58 wetlands in 1994 to determine mean brood sizes of black ducks and mallards in forested and agricultural landscapes. Study wetlands were moderately to highly fertile. We monitored 94 black duck broods each year and 46 (1993) and 52 (1994) mallard broods until they reached Class IIc-III (near fledging). No differences existed (P = 0.71) in mean brood size between black ducks (1993, 3.95 ? 0.23; 1994, 4.59 ? 0.24) and mallards (1993, 3.96 ? 0.35; 1994, 5.00 ? 0.43) either year. Brood size for species; however, was different between years (P = 0.014) and among wetland sites (P = 0.001). Mean sizes of broods were larger (P < 0.05) on 2 large impoundment complexes (Lake Josephine and Lake Christina) compared with brood sizes on other wetlands in forested or agricultural landscapes. No differences (P 0.41) existed between mean Class IIc-III, brood sizes of black ducks and mallards whether species were alone or together on wetlands. Our data document that mallard productivity is similar to that of black ducks where they breed sympatrically in Maine.