Conslusion Knowledge of the magnitude of both hunting and nonhunting mortality of waterfowl and of the causes of nonhunting mortality is needed to understand waterfowl population trends, to determine numbers of waterfowl available for harvest, and to develop effective management strategies. The relatively recent application of computer modeling to waterfowl investigations provides the means to integrate data on waterfowl mortality with other population data to reveal the importance of various mortality factors. However, for most species in most areas, data are insufficient to determine the magnitude of nonhunting mortality and its effect on populations. It is particularly difficult to detect and quantify waterfowl mortality during the breeding season, because the mortality tends to be inconspicuous and dispersed over vast areas.
Substantial biases can affect interpretation of mortality data.
The evidence we reviewed concerning determinations of hen mortality from waterfowl remains found at nests, hen success from use of apparent nest success data, and number of young fledged from brood counts that failed to account for total brood losses are examples of such serious biases. Radiotelemetry is an important tool that has expanded insight into the magnitude and causes of waterfowl mortality, but investigators using this and other new tools must be alert to possible new biases. Because most causes of mortality of waterfowl during the breeding season, especially of adults and young, have been subjectively determined, the interpretations of such data are subject to error. Until more definitive data on nonhunting mortality become available, the relative importance of various sources of mortality and their impacts on populations will continue to be debated. Mortality during the breeding season is a natural component of the annual cycle of fluctuating population sizes of waterfowl.