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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. 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. If his hypothesis is correct, the total production of young
ought to be highest at the normal clutch size. 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. 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. Question 1 of 30
Paragraph 1
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. 1.The word "ultimate" in the passage is closest in meaning to
A. fundamental
B. characteristic
C. various
D. immediate
Question 2 of 30
Paragraph 2
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. 2.Which of the sentences below best expresses the essential information in the
highlighted sentence in the passage? Incorrect choices change the meaning in important
ways or leave out essential information. A. Experimental evidence suggests that for most birds, the physiological limits of egg
production depend on the circumstances. B. Experimental evidence suggests that most birds are physiologically capable of laying more
eggs than they normally do. C. Most experiments yield similar evidence about the extent to which egg laying is normally
determined by the physiological limits of egg production. D. Although some experiments suggest that birds lay as many eggs as they can, other, similar
experiments suggest that this is not true for most birds.

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