1992. Comparative productivity of American black ducks and mallards nesting on Chesapeake Bay islands. Can. J. Zool. 70: [225][226][227][228]. We estimated laying dates, clutch sizes, and nest success rates of sympatrically breeding populations of American black ducks (Anas rubripes) and mallards (Anas platyrhynchos) on Chesapeake Bay islands between 1986 and 1989. Neither average laying date nor clutch size differed between black ducks and mallards. Nest success rates were higher for mallards in 2 of 4 years, but were area dependent. KREMENTZ, D. G., STOTTS, D. B., PENDLETON, G. W., HINES, J. E., et STOTTS, V. D. 1992. Comparative productivity of American black ducks and mallards nesting on Chesapeake Bay islands. Can. J. Zool. 70 : 225-228. Nous avons estime les dates des pontes, la taille des couvees et les taux de succks de la nidification chez des populations reproductrices sympatriques de Canards noirs (Anas rubripes) et de Canards colverts (Anas platyrhynchos) dans des iles de la baie de Chesapeake entre 1986 et 1989. La date moyenne de la ponte et le nombre moyen d'oisillons par couvee etaient les memes chez les deux espkces. Le taux de succks de la nidation s'est aver6 plus eleve chez les Canards colverts au cours de 2 des 4 annees, mais cela variait en fonction de la region.[Traduit par la rkdaction]
IntroductionThe potential causes of the long-term decline of the American black duck (Anus rubripes) (Rogers and Patterson 1984) are many and include habitat alteration and loss, overharvest, pollution, and competition/hybridization with the mallard (Anus plathyrhynchos) (Rusch et al. 1989). Coincident with the decline in black ducks has been a relative increase in mallards breeding in the Atlantic Flyway (Johnsgard and DiSilvestro 1976). Because there are few reported differences in the life histories of these species (Palmer 1976), factors negatively influencing black ducks might similarly influence mallards of eastern derivation. Yet in the Atlantic Flyway, mallards appear to be declining at a slower rate than black ducks (Strange et al. 1989). Nichols et al. (1987 showed that sympatric populations of mallards and black ducks did not differ in survival rates, and they therefore suggested that changes were attributed to differential immigration or natality. In this paper, we assess whether this difference in population trends might be due to differential natality.To test the hypothesis that mallards of eastern derivation produce more young than do black ducks, we studied a population of sympatrically breeding black ducks and mallards in the Maryland portion of Chesapeake Bay. The three variables we compared were laying date, clutch size, and nest success rate. If mallards have higher natality than black ducks, then (i) mallards should begin laying earlier than do black ducks, and (or) (ii) mallard clutch sizes should be larger than those of black ducks, and (or) (iii) mallard nest success should be greater than that of black ducks.