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.