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1. Components of the Population Dynamics

Good estimates of the probability that a sexually mature bird will breed and of the probability of renesting after failure (or success) of previous nesting efforts are few. Our modeling exercise demonstrated the significance of these quantities, but the values we used mostly lacked empirical foundations.

Clutch size and egg size have been well estimated for many waterfowl species, but the ultimate factors responsible for variation are not clearly understood. A number of proximate factors have been identified, however, and our knowledge of these components seems consistent with their importance to breeding dynamics.

Egg survival, specifically nest success, has been estimated for a number of populations during the past decade. Recent work has shown considerable geographic and temporal variation within a species, usually related to predator pressure and habitat conditions. The causes of this variation are worthy of considerable attention, particularly in view of the importance of egg survival to the breeding ground population dynamics of waterfowl. Specific questions involve determining the causes of clutch destruction and isolating the effect of nest density on survival probability.

There are relatively few good estimates of brood survival for waterfowl, in testament to the difficulty of studying the often-elusive young. Yet brood survival rates are critical in the dynamics; our modeling exercises placed them first or second in importance (after nest success) for the three populations considered. Further, the causes of brood loss are almost never known.

2. Inherent and Environmental Influences on the Components

Among the environmental influences, wetland conditions are most prominent, as they have been shown to affect all components. Nesting habitat certainly affects egg survival and perhaps other components. Weather exerts a strong influence on many components, notably brood survival, egg survival, and breeding incidence. The effect of population density has been suggested for brood survival but is less clearly indicated for most of the other components. This factor merits much more work, especially as management strategies may concentrate waterfowl in diminished habitats. Disease, parasites, and environmental contaminants have been pointed to as factors relevant to waterfowl population dynamics, but their roles on the breeding grounds are poorly understood.

3. Study Approaches

Experimental studies are also needed to supplement findings from observational approaches. There are too many correlated variables in natural situations to sort out the individual effects of specific variables. Some of the more exciting findings of recent years were based on innovative research designs, a trend we strongly endorse. Some experimental studies require the use of captive animals. Care should be taken to simulate natural conditions as closely as feasible, and results from captive situations should be confirmed in natural populations before firm conclusions are reached.