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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. Were remains of waterfowl found at a fox den the result of predation or scavenging? Was a bird killed by a predator healthy or predisposed to death by another mortality agent? Would a contaminated, diseased, or weakened bird that was killed by a predator have survived if it had not been preyed on? 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. However, human influences on certain populations, through both environmental alterations and harvests, have changed the magnitude, causes, and importance of waterfowl mortality during the breeding season. Nonhunting mortality during the breeding season has resulted in major reductions of annual production below desired levels in numerous situations.

We conclude that predation is the major source of mortality affecting most North American waterfowl during the breeding season. The greatest effect of this mortality agent appears to be on survival of eggs, although predation rates on females (especially ducks) and prefledged young are often severe. Although predation is a natural mortality agent, it is also an unstable force, and its effect on populations is easily altered by changes in local environments. It can be argued that other stressors and mortality agents (e.g., weather, contaminants, diseases) contribute to high predation rates, but evidence to support such a hypothesis is meager. Moreover, we have referenced numerous examples where alterations of predation rates alone, generally through human interventions, resulted in substantial changes in waterfowl population sizes and nesting success.

For certain populations of geese in arctic regions, subsistence hunting is a form of human predation that ranks high in importance as a mortality agent. The combined effects of mortality from predation on local waterfowl populations already reduced by subsistence hunting or any other factor can be particularly severe. Because of the demonstrated powerful effects of both natural and human predation on many waterfowl populations and the fact that these agents are amenable to management, increased attention is being given to reducing effects of predation on waterfowl (e.g., Braun et al. 1978, Lokemoen 1984, Sargeant and Arnold 1984, Pamplin 1986, Johnson et al. 1987). Such efforts are often controversial, especially if they involve predator removal or restrictions of native harvests, but they warrant serious consideration. If humans have altered relationships between predators and prey so as to result in inordinately high levels of waterfowl mortality, is it not proper to consider establishing a new balance that favors restoring higher waterfowl populations?

Weather also ranks high in importance as a mortality agent affecting waterfowl during the breeding season, especially in higher latitudes. The most important effect of weather is probably on mortality of young during the first 2 weeks after they hatch. Chilling has often been implicated in such losses, but its effect on waterfowl populations is far less certain than that of predation. Weather, of course, cannot be manipulated to benefit waterfowl, but its true influence and potential synergistic effects with disease and other mortality agents need study.

There is considerable concern over the potential for contaminants and diseases to have major impacts on waterfowl populations. However, except at relatively few locations, neither agent is known to have inflicted severe mortality on North American waterfowl during the breeding season. Mortality from these causes could go undetected, because many affected birds might be killed by predators before dying from contaminants or disease. Nevertheless, it is unlikely that major die-offs of adult waterfowl in populated areas would go undetected. Hatchability of waterfowl eggs tends to be high unless the eggs are abandoned or destroyed, indicating that contaminants and diseases have had little effect on egg mortality.

The greatest known or suspected effects of contaminants and diseases are on survival of prefledged young, which suffer high mortality from largely undetermined causes. A major concern regarding contaminants and diseases is that waterfowl repeatedly exposed to these agents may have reduced reproductive potential and increased risk to mortality from other agents. There is also concern that contaminants may reduce waterfowl food supplies in certain areas, thereby placing additional stress on young waterfowl and increasing risk of mortality from other agents.

Mortality agents other than contaminants, diseases, predation, subsistence hunting, and weather appear to have minor effects on waterfowl populations. At some sites a specific agent such as collisions with power lines or hay-cutting machinery may significantly reduce a local population. Although the overall mortality from these other agents appears to be relatively low, these losses may sometimes be easily prevented in local areas.

Waterfowl populations face continued habitat losses and other negative human influences that act to lower population sizes. Waterfowl are at high risk of mortality during the breeding season and realize only a small fraction of potential annual population increases. The future management of waterfowl populations will require more accurate enumeration of the magnitude and causes of mortality throughout the year than have been available in the past and a better understanding of the relative vulnerability of various population cohorts (e.g., sex, age, social status, breeding status) to major mortality agents. A challenge for the future will be to prevent unnecessary mortality and effectively manage the dwindling, and increasingly altered, habitat base.