A review of seasonal declines in clutch size of waterfowl (Anatidae) in the northern and southern hemispheres

Abstract

The size of the clutch declines with the date of laying of the first egg in many, but not all waterfowl (Anatidae) . This paper reviews the published data on rates of decline in clutch size of waterfowl during the breeding season to : (i) determine whether the decline rates were related to the length of the breeding season at the nesting site, and/or to whether the birds were nesting in captivity or in the wild and; (ii) to propose tests of the reasons that have been suggested previously to account for these seasonal declines in clutch size. Rates of decline were greater at higher latitudes ( 50°) (where breeding seasons are relatively short) than at middle latitudes (30° - 49°) (where breeding seasons are longer). The majority of waterfowl that exhibited no or low rates of decline nested at middle latitudes, or in captivity . These findings are consistent with the three main factors (seasonally declining nutrient reserves, physiological effects ofdiminishing daylength, seasonally diminishing chances of survival) that have been proposed as causes of intra-seasonal declines in clutch size ofwaterfowl. Feeding studies, relocation studies, and experiments in which light regimes are manipulated, are required to distinguish between these threepossible causes of falling clutch size with laying date in waterfowl.