Hunting among the forces that shape duck behavior

Some species are more likely to be negatively affected by pressures of hunting season

Posted: Sunday, October 29, 2006

There can't be very many towns in North America that have a sizable wetland and wildlife refuge right in their midsections. We have this extraordinary resource that is accessible to almost everyone and used in different ways by many folks - and by lots of wildlife.

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The Mendenhall Wetlands provide feeding and resting areas for many species of waterfowl, including geese, diving ducks and sea ducks, and dabbling ducks. Dabblers often feed by tipping down with their heads under water and their tails in the air and by picking small invertebrates and seeds from the surface and adjacent vegetation.

The most common dabbling ducks on the wetland are mallards, with smaller numbers of green-wing teal, northern pintails, northern shovellers, American wigeon and gadwall. Dabbling ducks are most numerous on the wetland in April and May, as migrants stop over to refuel and rest for their continuing journey to breeding areas.

Large numbers of mallards also spend the fall and winter on the wetland, where they are widely distributed, and there are smaller numbers of the other species. These ducks do more than feed and rest to achieve good body condition for the coming breeding season. Fall and winter are the seasons when these ducks compete intensively for mates, court and form pairs.

These social activities cost a lot of energy, requiring a high food intake. So the choice of good feeding areas is important for these ducks. Biologists and hunters know ducks differ in wariness - some species are willing to take more risks in approaching a potential feeding area, while others are more cautious.

A major risk faced by ducks in fall and winter comes from hunters: Hunting is reported to account for about half of all annual mortality of ducks. Such strong pressure has the potential to change the risk-taking behavior of the birds.

A recent article in one of the professional ornithological journals shows that risk-taking is related to the life history of dabbling ducks, including the species on the Mendenhall wetland. Risk-taking was measured by the willingness of each species to approach standard decoys during the hunting season in California.

Relatively small ducks (shoveller, teal) have low annual survival (46 to 48 percent) and larger ducks (pintail, mallard, wigeon, gadwall) have higher annual survival (58 to 63 percent). The smaller species also have high fecundity in proportion to body size, or "reproductive effort": females of the smallest species produce clutches of eggs that weigh 62 to 70 percent of their body weight, but the clutches of the largest species are only 36 to 41 percent of the female's weight.

In other words, ducks with a small chance of living more than a year or two tend to invest more in reproduction, because they may not get another chance. The researchers found that the species with the highest reproductive effort were the biggest risk-takers (most willing to approach the decoys in hunting season), and those with the lowest reproductive effort were the most cautious. Thus, short-lived ducks may shorten their lives still further by taking risks that would improve their foraging opportunities and their reproductive output (if they live).

There are several implications of these findings. Ducks with the highest reproductive effort (and lowest survival) can least afford to miss feeding opportunities and may have to take more risks, perhaps resulting in even lower annual survival. Lower survival, in turn, commonly requires still higher reproductive effort, if the population is to remain stable. If higher reproductive effort is not possible (for any number of reasons, such as limits to clutch size or decreases in survival that are too rapid for adaptation in reproductive effort to occur), the population could decline.

Predation by humans is known to produce evolutionary changes in their prey populations. Selective harvest of large salmon, for example, can lead to a reduction of body size of salmon in subsequent generations. By removing the large animals from the population, the smaller ones are given more opportunity to breed, so their genes become more common in the population and eventually (to the extent that size is controlled genetically) the animals in the population become smaller.

Game and fish management sometimes takes account of such human-induced changes. There are many other aspects of the biology of prey species (beyond body size), however, that can change in response to human predation, and this clearly affects what management practices will work best in the long term, as well as opportunities for both natural history observations and hunting.

• Mary Willson is a retired professor of ecology and a board member of Trail Mix.

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