Why are some ducks are relatively abundant and widespread while others are never numerous or widely distributed? Why can some species like the mallard perennially sustain heavy hunting pressure while others like the canvasback sometimes require full protection from the gun? What factors determine growth in duck populations?

These are questions most of us have wondered about, especially those of us who have hunted long enough to have experienced years when ducks were bountiful, along with seasons when flights were lean.

Three key factors play pivotal roles in the growth of populations – the reproductive potential of a species, the quantity and quality of its breeding habitat, and the size of the breeding population, particularly the female component. The first factor is set by evolution but the other two are dynamic, varying from year to year, sometimes dramatically. When abundant populations of a species with high reproductive potential encounter good breeding conditions, growth can be exceptional. Conversely, when habitat conditions are poor, population growth is compromised, even in abundant populations of species with high reproductive potential. Understanding the relationship that exists among these three factors is the basis for setting harvest goals and hunting regulations.

The fact that ducks can live for a relatively long time – potentially 10 to 12 years, though usually much less – is very important. In general larger species have greater longevity than smaller ones, so life span depends on the species and the circumstances of the individual. Living a long time is an evolutionary adaptation that allows ducks to cope with years of poor reproductive success due to adverse habitat conditions on the breeding grounds. The quality of breeding habitat varies in dynamic environments and this affects individual success and population growth.

Ducks have adapted to many breeding habitats. Because they are strong flyers, they can travel a considerable distances to find good conditions, which more than anything else means food. Indeed, this is the evolutionary thrust behind migration. Ducks attempt to breed where food is available to raise their young, if only temporarily.

Ducks also have evolved diverse reproductive strategies. Some produce many young, others only a few. In general, dabbling ducks have a higher reproductive potential than diving ducks, and divers have a greater reproductive potential than sea ducks like scoters and eiders. More than any other factor, it is robust reproductive potential that explains how ducks can be so abundant and also recover so quickly when populations have declined. This is also why hunters can annually enjoy an open season. Few other birds could sustain their populations in the face of the kind of hunting pressure we impose on ducks.

Reproductive potential can be best illustrated with dabblers. Pintails and shovelers represent the extremes, with shovelers having less potential to produce large numbers of young than pintails. Gadwalls and widgeon are closer to shovelers, while mallards are nearer to pintails. Black ducks, though closely related genetically to the mallard, are closer to the shoveler than the pintail.

Notwithstanding potential reproductive capability, habitat conditions play the controlling role in reproductive success. Poor nesting and brood-rearing conditions can thwart reproduction, even of females of species that can potentially produce many young. This has obvious implications for population growth.

Pintails and shovelers are good examples of the relationship between reproductive potential and habitat conditions. The pintail is a habitat opportunist that breeds in diverse habitats but is most productive when it breeds on the prairie. Typically pintails arrive early in the spring and quickly initiate nesting. In this way they take advantage of temporary water, which provides an abundant but short-lived food supply that is critical for duckling growth. However, ducklings must be fledged before this water disappears. Pintails and many other dabblers may be capable of nesting again if they are initially unsuccessful. However, initial efforts usually produce the greatest number of young.

Focusing on temporary water is risky. The costs of being so dependent on temporary water can be disastrous, but the benefits can be remarkable. On a drought-prone prairie, pintails may be able to breed in years when species that nest later have insufficient water to raise their broods.

Prairie pintail populations track wet and dry cycles. In good years their robust reproductive potential can be maximized, resulting in a population boom. Conversely, during dry years production is usually poor. However, because pintails can live a long time, individuals and populations can cope with protracted drought. These adaptations to prairie habitats, coupled with high reproductive potential and large populations, have allowed pintails to reach numbers second only to the mallard among prairie nesting dabbling ducks.

On the other hand, shovelers seek quite different breeding habitats. They arrive on the breeding grounds several weeks later than pintails. They utilize relatively permanent water which typically provides less food for ducklings than the rapidly drying wetlands used by pintails. But these more stable wetlands usually can be relied upon to last until the shoveler ducklings fledge. In most years, young pintails are on the wing several weeks before shovelers.

This has implications to the population. The habitat used by shovelers will usually allow some reproductive success, even in dry years. This is fine for these birds that have modest reproductive potential. Although the shoveler cannot take advantage of exceptional habitat conditions and experience booms in growth like pintails, they rarely experience busts. Their populations tend not to fluctuate as sharply as pintails. Shovelers did not seriously decline during the drought of the 1980s and early 1990s, but neither did their populations grow dramatically when the drought broke.

Mallards are the most abundant and widely distributed of any duck. They combine high reproductive potential with an amazing ability to adapt to a diversity of breeding habitats throughout the northern hemisphere. They successfully pioneer new habitats but nowhere are they better able to maximize their superior reproductive potential than on the North American prairies. In fact it is on the “duck factory” that many species maximize their reproductive potential and breed in the highest densities. The wonderfully adapted mallard has become the consummate prairie duck and provides a good example of how both female success and population growth are a reflection of habitat quality.

While mallards are reasonably successful everywhere they breed, from urban parks to the most remote wilderness habitat, like the harsh saline habitats of coastal Greenland, their breeding success and population growth in these areas does not rival that of the prairie. Mallards also breed in the forested habitats of the northeast, the stronghold of the closely related black duck. However, their reproductive success and population growth is relatively modest. The forested habitat does not provide a food base comparable to the prairie, therefore northeast mallard populations will never rival their prairie counterpart. The lack of food is almost certainly the reason why the black duck has never been a particularly abundant species.

One habitat that rivals the prairie for mallard production is in New Zealand, where the mallard was introduced. After initial difficulties, a breeding population became established and eventually flourished. Unlike North American habitats where ducklings are raised in wetlands, parts of New Zealand possess few such wetlands. Females raise their young on pastureland where there is little if any standing water. Sheep paddocks (pastures) provide as high quality rearing habitat as the North American prairies. The success of these sheep-paddock mallards is reflected in the hunting regulations – seasons last several months and in places the daily limit is 50 birds. New Zealand may legitimately lay claim to being the mallard hunting capital of the world, and it provides an excellent example of the mallard’s adaptability.

Scaup pose a different problem. The duck once ranked second to the mallard in abundance. It has relatively good reproductive potential. But in recent times it has failed to recover from historically low populations. This failure is probably different for the two species of scaup. The lesser scaup is a prairie breeder that must contend with degraded habitats. The greater scaup breeds in the boreal forests to the north. This habitat has a much lower productive capacity, but its vastness combined with large numbers of breeders have traditionally sustained populations. For both the lesser and greater scaup, it may be that their numbers have fallen to the point where recovery is seriously jeopardized even when habitat conditions might be expected to result in good production.

Pintail and scaup have historically increased dramatically when prairie potholes filled with water. However, even a wet prairie today has lost much of its ability to produce ducks. Degraded habitats simply do not allow ducks to maximize their reproductive potential.

The low populations of recent times also may result from management attitudes that are entrenched in a bygone era when water on the prairie meant explosive increases in numbers of ducks. Under the most favorable habitat conditions today, there may be too few breeders to allow numbers to dramatically increase. It may be that waterfowl management allowed too many birds to be shot in the mistaken belief that a wet cycle on the prairie would cause populations to rebound. Scaup and pintails have not experienced a boom year in decades.

The canvasback is a good example of a duck that has always required careful monitoring, often requiring complete protection from the gun. It has modest reproductive potential and it is more reliant than many other species on specialized habitat for breeding success. There is a good chance that cans never really recovered after heavy gunning and the 1930s prairie drought combined to depress their populations. Even with protection from the gun, there is little likelihood that cans will ever increase much beyond what they are today, given the poor quality of nesting habitats and low breeding populations.

Importantly, the U.S. Fish and Wildlife Service uses the mallard as an indicator of duck population trends. This is inherently risky, because the adaptable, resilient mallard is going to achieve some measure of breeding success even under adverse habitat conditions. This is one reason they have been able to sustain perennially heavy hunting pressure. Mallards may do reasonably well while other species like pintails and scaup have disastrous breeding seasons, a fact that becomes fully apparent to hunters and managers only after the hunting seasons are set and shooting commences, too late to offer the troubled species greater protection from over-harvest.