The Shotgun Approach to Nest Success

Did you ever wonder why some species have a lot of offspring and provide only short-term limited care for their young, while others have few offspring and provide long-term intensive care? These differences in reproductive strategies have been the object of scientific investigation and debate for years.

Some scientists believe that animals living in variable environments (those subject to periodic droughts, etc.) can maximize their reproductive success by having numerous young when conditions are favorable, so that at least some will survive. When conditions are poor, these same species reduce reproductive output by either choosing not to breed that year or by limiting their breeding attempts. In contrast, for animals that live in more stable environments, more of their offspring would be expected to survive, and their energy is better spent raising only a few very healthy young who can successfully compete with others.

Taking a bit of a different angle, other scientists have argued that animals at the bottom of the food chain must have many offspring to compensate for the large numbers lost to predators, resulting in a large reproductive effort each year. This is purely a numbers game: Get enough young out there, and chances are good that some will survive the gauntlet.

Prairie-nesting ducks have evolved under both of these selective pressures, variable environments and high rates of predation. In ducks, one visible result of natural selection under high rates of predation is the drab coloration of females. Their drab plumage has been genetically shaped over many years, allowing hens to blend with the grasses and sedges where they place their nests, so that they are less conspicuous to predators.

Although there is variation among species, female ducks also lay about 10 eggs per nesting attempt, and most species show a strong tendency to renest if their first nest is destroyed. The overall strategy is to get as many eggs out there as they can when conditions are good, in the hopes that at least some will make it.

Scientists also believe that the behavioral and physical characteristics that have the greatest impact on reproductive success (i.e., the ability to pass on one's genes to the next generation) will undergo the strongest selective forces. Our current understanding of waterfowl population dynamics strongly suggests that nesting success and female survival during the breeding season are the two factors most limiting waterfowl reproductive success and population size. Therefore, natural selection should favor those traits and behaviors that increase nest success and survival. For example, brighter plumage in females might improve the frequency of courtship and pairing, but at the same time it would increase detection of nesting females by predators. If survival during the breeding season (drab coloration) is more limiting to reproductive success than early courtship and pairing (bright coloration), drab females will be more successful and pass their genes down to subsequent generations.

Today, waterfowl nest success across the prairies averages 15 percent, and only 72 percent of females survive the breeding season. This results in only 5 percent of all eggs that are layed surviving to produce a flying duck in the fall. What is interesting, though, is that these nest success percentages do not appear to be drastically lower than historic levels. Scientists estimate that before most of the wetlands were drained and the grass plowed under, waterfowl only experienced nesting success around 40 percent. No wonder females are drab and lay so many eggs. Predators have always been good at finding a needle in the haystack, and waterfowl have evolved strategies and behaviors to allow for this, while at the same time maintaining viable population levels.

The problem today is that the haystack has gotten much smaller. Ninety percent of the grasslands and 70 percent of the wetlands on the prairies have been lost. Under historic prairie circumstances, the prey community was undoubtedly very diverse, with predators choosing prey among several species of birds and small mammals such as rabbits, mice and voles. (Natural selection also worked to shape the appearance of all these species to improve their ability to avoid being detected.) But waterfowl today have fewer wetlands and grasslands to nest in, and the fragmentation of this habitat has also changed the predator community from one dominated by wolves and coyotes, which focus preying on small mammals, to one dominated by foxes, raccoons and skunks, species that tend to focus on ducks and their eggs.

Biologists also speculate that fewer small mammals such as mice and voles are available in small grassland fragments. The availability of this alternate prey can have significant impacts on waterfowl nest success. A recent study in California found that nest success was much higher in fields with abundant mice and voles across similar amounts of nesting cover. Experiments with several species of animals have documented that when looking for food, individuals choose situations that provide the highest probability for reward, and that they are able to learn from past and present experiences.

Based on this theory, prairie predators encountering mice and voles at high rates might shift their search patterns to focus on these prey items instead of nesting ducks and their eggs. In contrast, when mice and vole populations are low, predators may become more focused on duck nests. Unfortunately, our understanding of what drives mice and vole population dynamics on the prairies and the relationships of these small mammal populations to waterfowl nest success is lacking.

Despite all these changes, waterfowl continue to adapt as best they can. More birds are now settling in the prairies of the United States, where the Conservation Reserve Program and efforts of the U.S. Fish and Wildlife Service, Ducks Unlimited and other partners have provided extensive grasslands for upland-nesting ducks. In contrast to historic patterns, the U.S. prairies are now producing as many young each year as their prairie habitat counterparts in Canada. Current research in the U.S. prairies sponsored by DU shows that on landscapes containing 70 percent or more grassland, duck nest success rates are averaging 32 percent, with several at the 40 percent level, which scientists speculate was common before the prairies were so drastically altered. Compare this to 16 percent on landscapes containing less than 70 percent grassland. Enlarging the haystack not only makes nests more difficult to find, but it also favors a predator community dominated by coyotes and badgers instead of foxes, skunks and other species that favor preying on ducks and their eggs.

Predators and their prey have been involved in evolutionary warfare for eons, each changing and adapting to either improve efficiency at detecting and capturing prey, or improve efficiency at hiding from and escaping predators. Given time and a fairly natural environment, they have been able to strike a balance that allows populations of both to persist, although they may wax and wane with changes in weather and local habitat conditions.

The drab coloration of female ducks, selection of nest sites in dense upland cover, large number of eggs and propensity to renest have allowed waterfowl populations the ability to coexist with predators on the prairies for thousands of years. Over long periods of time, prey have been able to stay ahead of predators in the evolutionary arms race because they have a shorter generation time and, therefore, can genetically adapt faster. But, recently, man has changed this landscape drastically, quickly shifting the balance away from waterfowl and toward predators.

Our challenge is to protect and restore enough grassland in the right places to allow enough nests to hit their mark in favorable cover, so that waterfowl populations can be sustained well into the future. We owe these majestic and resilient creatures nothing less.