Accounting for Recruitment
Although we can speculate about how many breeding ducks the prairies once supported, this doesn't tell us how big duck populations actually got. Most duck hunters are familiar with the term "fall flight
," the size of the duck population going into the fall. Fall flight estimates are based on the size of the breeding population, habitat conditions, and the number of young produced that survive into fall. Since the 1950s, years with good breeding conditions on the prairies and elsewhere have produced fall flights of 100 million birds or more.
That brings us to the topic of "recruitment." Waterfowl managers define recruitment as the number of young added to the population by reproduction from adults in the spring. Recruitment depends on the percentage of hens that successfully hatch a nest—or hen success—and duckling survival. To answer the fall flight question, we need to consider possible changes in these variables.
Hen success is a function of nest success and renesting intensity. Nest success
is simply the probability that a nest will hatch. Renesting intensity is strongly dependent on breeding habitat conditions and varies considerably among species, with mallards being the most likely renesters. Hens renest more often in wet years because more habitat is available to support late-hatched ducklings. We don't know if the relationship between renesting intensity and breeding habitat conditions has changed over time, but ducks still renest more frequently when it's wet and less frequently when it's dry. Consequently, we assume that long-term changes in hen success are probably due mostly to changes in nest success.
There is some evidence that nest success on the prairies declined by as much as 50 percent between the 1930s and the 1970s. But nesting ducks have never had it easy. Hen mallards marked with radio transmitters have renested as many as five times. This behavior evolved thousands of years before the prairies were cultivated and strongly suggests that nest predation was always high. To be conservative we'll assume that nest success before settlement was double that of today.
Changes in duckling survival are the last piece of the recruitment puzzle. Although there are no long-term studies of duckling survival, we do know that mink are a major duckling predator. Mink numbers crash during times of drought, though some mink are able to persist where permanent water is available. Irrigation projects and cattle operations have probably increased the amount of permanent water on the prairies, which may allow mink populations to quickly rebound when wet weather returns. As the prairies emerged from drought, presettlement duck populations may have enjoyed a longer holiday from mink predation than the birds do today.
Habitat loss on the prairies may also have reduced duckling survival. Hens frequently move their broods from wetland to wetland because of changes in food availability. With fewer wetlands on the landscape, hens and ducklings often have to move farther overland in search of food, which increases their exposure to upland predators. In fact, research indicates that duckling survival is higher on landscapes with more seasonal wetlands. As a result, the widespread loss of seasonal wetlands on the Canadian and U.S. prairies may have reduced duckling survival while also decreasing the number of breeding pairs.