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The Shotgun Approach to Nest Success

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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.

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