Today, it is hard to imagine that winter foraging habitat limits the size of North American goose populations, though some species like brant are still largely confined to coastal habitats. But what about ducks? It is possible to argue that some species of ducks were historically limited by winter habitat, at least in some years. Consider an unbroken prairie in times of abundant rainfall when fall flights might have easily topped 200 million birds. Now consider these birds arriving in the Mississippi Alluvial Valley (MAV), Gulf Coast, or Central Valley to find these areas in the grip of drought. With no managed habitat or waste grain, heavy competition for limited food resources may have resulted in high rates of mortality. In addition, birds that did survive the winter may have experienced lower reproductive success the following spring.
What about the current role of winter habitat in regulating North American duck populations? One thing seems clear: Wintering ducks do better in years when foraging habitat is abundant. For example, mallards in the MAV are heavier in years of above- average fall and winter rains. More rain in the MAV results in more habitat, which ultimately increases food availability and allows mallards to more easily store body fat. In a similar manner, northern pintails in the Central Valley of California weigh more in years of increased precipitation.
Heavier birds in years of abundant precipitation provide circumstantial evidence that winter food may play a role in regulating the population sizes of ducks. The key word here is circumstantial. To fully understand the impact of winter habitat on duck populations, we need to quantify what effect changes in winter habitat availability have on survival and reproduction, and how this varies on an annual basis.
There is some evidence that winter habitat does influence survival through its effects on body condition. For example, canvasbacks in the Chesapeake Bay that had high winter body weights survived better than birds of lower body mass. Given that body weights of ducks seem to decline in winters when habitat is reduced, the results for canvasbacks suggest that reductions in winter habitat can lower survival rates.
What about the effects of winter habitat on reproduction the following spring? In the early 1980s, researchers in Missouri examined the relationship between the abundance of winter habitat and mallard reproduction. Years with above-average habitat conditions corresponded to greater mallard breeding success the following year. This apparent link between winter habitat conditions and reproduction led to tremendous interest in events outside the breeding grounds, and was a landmark in waterfowl research. Later work on northern pintails also suggested a relationship between winter habitat and changes in population size.
Despite the difficulty in linking events on the wintering grounds to the size and success of breeding duck populations, there does seem to be a relationship. The real debate continues to be just how strong these relationships are. In reality, we may never be able to truly quantify the effects of winter habitat on survival and reproduction.
However, we can be sure of one thing: Winter habitat will limit North American waterfowl populations at that point when none of it remains.