Clutch Size in Birds

Each year the emperor penguin lays one egg, the pigeon lays one or two eggs, the gull typically lays three eggs, the Canada goose four to six eggs, and the American merganser ten or eleven eggs. What determines clutch size in birds? The ultimate factors that determine clutch size are the requirements for long-term (evolutionary)survival. Clutch size is viewed as an adaptation under the control of natural selection.

Natural selection will favor those birds that leave the most descendants to future generations. At first thought we might hypothesize that natural selection favors a clutch size that is the physiological maximum the bird can lay. We can test this hypothesis by taking eggs from nests as they are laid. When we do this, we find that some birds, such as the common pigeon, are determinate layers; they lay a given number of eggs, no matter what. The pigeon lays two eggs, if you take away the first, it will incubate the second egg only. If you add a third egg, it will incubate all three. But many other birds are indeterminate layers; they will continue to lay eggs until the nest is "full". If eggs are removed once they are laid, these birds will continue laying. When this subterfuge was used on a mallard female, she continued to lay one egg per day until she had laid 100 of them. Evidence from other, similar experiments suggests that most birds under normal circumstances do not lay their physiological limit of eggs but that ovulation is stopped long before this limit is reached.

The British ornithologist David Lack was one of the first ecologists to recognize the importance of evolutionary thinking in understanding adaptations in life history traits. [■A]In 1947 Lack put forward the idea that clutch size in birds was determined by the number of young that parents can provide with food. [■B]If his hypothesis is correct, the total production of young ought to be highest at the normal clutch size. [■C]And if one experimentally increased clutch size by adding eggs to nests, increased clutches should suffer greater losses because the parents could not feed the extra young in the nest. [■D]

One way to think about this problem of optimum clutch size is to use a simple economic approach. Everything an organism does has some costs and some benefits. The benefits of laying more eggs are very clear--more descendants in the next generation. The costs are less clear. There is an energy cost to make each additional egg, and there is a further cost to feed each additional nestling. If the adult birds must work harder to feed their young, there is also a potential cost in adult survival -the adults may not live until the next breeding season. If adults are unable to work harder, there is a risk of reduction in offspring quality. Models of this type are useful because they help us think about the costs and benefits for a particular ecological strategy. No organism has an infinite amount of energy to spend on its activities. The reproductive rate of birds can be viewed as one sector of a bird's energy balance, and the needs of reproduction must be maximized within the constraints of other energy requirements. The total requirements involve metabolic maintenance, growth, energy used for predator avoidance, competitive interactions, and reproduction. According to Lack's hypothesis, if enough additional eggs are placed in a bird's nest, reproductive energy requirements escalate and the whole brood will suffer from starvation so that, in fact, fewer young birds will fledge from nests containing large numbers of eggs.

In England, the blue tit normally lays a clutch of nine to eleven eggs. What would happen if blue tits had a brood of twelve or thirteen? A researcher artificially manipulated broods at hatching by adding or subtracting chicks and found that the survival of the young blue tits in manipulated broods was poor. Blue tits feed on insects and apparently cannot feed additional young adequately, so more of the young starve. Consequently, it would not benefit a blue tit in the evolutionary sense to lay more eggs and the results are consistent with Lack's hypothesis. Individual birds appear to produce the clutch size that maximizes their reproductive potential.

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Lack's proposed dependence of clutch size on food availability has clear implications..

Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This questions is worth 2 points. Drag your answer choices to the spaces where they belong. To remove an answer choice, click on it To review the passage, click VIEW TEXT. A bird species' normal clutch size is determined through evolutionary pressure to maximize its long-term reproductive potential.