By Mark Petrie, Ph.D., DU Research Scientist
Habitat management for wintering waterfowl dates back 130 years to when California established the first state refuge at Lake Merritt, Oakland. As early as the 1930s, state and federal wildlife agencies began efforts to protect winter habitat in all four flyways using duck stamp funds, license fees, and excise taxes on sporting arms and ammunition. To a great extent, then, our efforts to manage waterfowl habitat originated south of the breeding grounds.
Despite a long tradition of meeting the needs of wintering ducks and geese, it's not clear how conditions on the wintering grounds influence the size of North America's waterfowl populations. At the heart of the debate is food. During winter, food energy is the key requirement for ducks and geese. In theory, food availability on the wintering grounds can influence mortality and reproduction, the linchpins for population growth.
For example, birds faced with food shortages in winter may be more susceptible to the effects of natural predation, hunting, and disease. From a reproductive standpoint, a lack of winter habitat may prevent some birds from storing body fat and protein that is used in spring to produce eggs and meet the energy demands of incubation. The result is that winter food shortages could result in fewer birds returning to the breeding grounds, as well as reducing the success of birds that do breed.
That's the theory. But what real evidence do we have that conditions outside of the breeding season play a major role in regulating North American waterfowl populations? Let's start with geese. Historically, some species like snow geese were confined to narrow bands of coastal salt marsh during winter.
Intensive foraging within these limited habitats resulted in depletion of food resources prior to spring migration. As a result, population size was limited by overwinter survival, because many birds succumbed to the effects of food shortages. In addition, many of the birds that did survive the winter likely returned to the breeding grounds in poor shape and thus reproduction suffered.
Agriculture changed all that. As farming practices intensified, geese gained access to tremendous amounts of waste grains throughout winter and during migration. This energy subsidy reduced competition for food resources and increased survival rates outside of the breeding season. Moreover, birds were better able to store the body fat and protein used in reproduction. Agriculture not only increased the number of geese that returned to the Arctic, it has also increased the reproductive success of those birds.
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.