Historical Changes in Laying Date, Clutch Size, and Nest Success of American Black Ducks

Abstract

The breeding population of American black ducks (Anas rubripes) on Chesapeake Bay has declined over the past 30 years. We tested whether there have been changes in laying date, clutch size, and nest success of breeding black ducks on islands in Chesapeake Bay between the 1950's and 1980's. None of these variables differed significantly between decades, suggesting that other factors must be responsible for the decline in locally breeding black ducks. J. WILDL. MANAGE. 55(3):462-466 Many factors might contribute to declines in populations of American black ducks, including habitat alteration and loss (Dennis et al. 1985, Diefenbach and Owen 1989), overharvest (Geis et al. 1971, Grandy 1983), pollution (Longcore and Stendell 1982, Haramis and Chu 1987), and competition and/or hybridization with mallards (A. platyrhynchos) (Johnsgard and DiSilvestro 1976). Any of these could conceivably affect the breeding success of black ducks. Populations of black ducks have declined on Chesapeake Bay since the 1950's (Breeding Bird Surv., Off. Migr. Bird Manage., U.S. Fish Wildl. Serv., unpubl. data). We tested whether laying dates, clutch sizes, and nest success rates of black ducks differed between the 1950's and 1980's by comparing recent data with data collected by Stotts and Davis (1960) in the same area. Laying date was examined not only because it partially explains seasonal changes in clutch size (Klomp 1970), but also because it partially explains the seasonal decline in duckling survival rates (Orthmeyer and Ball 1990). We thank landowners granting us access to their lands. We also thank C. L. Holmquist and J. W. Gill for assistance in the field, G. W. Pendleton for assistance with the statistical analyses, and J. R. Serie, G. R. Hepp, I. J. Ball, and D. J. Forsell for valuable comments on early drafts. Funding was partially provided under PittmanRobertson Project Number W-30-R.