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Bird Colonies
About 13 percent of bird species, including most seabirds, nest in colonies. Colonial nesting evolves in response to a combination of two environmental conditions: (1) a shortage of nesting sites that are safe from predators and (2) abundant or unpredictable food that is distant from safe nest sites. First and foremost, individual birds are safer in colonies that are inaccessible to predators, as on small rocky islands. In addition, colonial birds detect predators more quickly than do small groups or pairs and can drive the predators from the vicinity of the nesting area. Because nests at the edges of breeding colonies are more vulnerable to predators than those in the centers, the preference for advantageous central sites promotes dense centralized packing of nests.
The yellow-rumped cacique, which nests in colonies in Amazonian Peru, demonstrates how colonial birds prevent predation. These tropical blackbirds defend their closed, pouchlike nests against predators in three ways. First, by nesting on islands and near wasp nests, caciques are safe from arboreal mammals such primates. Second, caciques mob predators (work together as a group to attack predators). The effectiveness of mobbing increases with group size, which increases with colony size. Third, caciques hide their nests from predators by mixing active nests with abandoned nests. Overall, nests in cluster on islands and near wasp nests suffer the least predation.
Coordinated social interactions tend to be weak when a colony is first forming, but true colonies provide extra benefits. Synchronized nesting, for example, produces a sudden abundance of eggs and chicks that exceeds the daily needs of local predators. Additionally, colonial neighbors can improve their foraging by watching others. This behavior is especially valuable when the off-site food supplies are restricted or variable in location, as are swarms of aerial insects harvested by swallows. The colonies of American cliff swallows, for example, serve as information centers from which unsuccessful individual birds follow successful neighbors to good feeding sites. Cliff swallows that are unable to find food return to their colony, locate a neighbor that has been successful, and then follow that neighbor to its food source. All birds in the colony are equally likely to follow or to be followed and thus contribute to the sharing of information that helps to ensure their reproductive success. As a result of their enhanced foraging efficiency, parent swallows in large colonies return with food for their nestlings more often and bring more food each trip than do parents in small colonies.
To support large congregations of birds, suitable colony sites must be near rich, clumped food supplies.
Colonies of pinyon jays and red crossbills settle near seed-rich conifer forests, and wattled starlings nest in large colonies near locust outbreaks. The huge colonies of guanay cormorants and other seabirds that nest on the coast of Peru depend on the productive cold waters of the Humboldt Current. The combination of abundant food in the Humboldt Current and the vastness of oceanic habitat can support enormous populations of seabirds, which concentrate at the few available nesting locations. The populations crash when their food supplies decline during El Nino years.
Among the costs, colonial nesting leads to increased competition for nest sites and mates, the stealing of nest materials, and increased physical interference among other effects. In spite of food abundance, large colonies sometimes exhaust their local food supplies and abandon their nests. Large groups also attract predators, especially raptors, and facilitate the spread of parasites and diseases. The globular mud nests in large colonies of the American cliff swallow, for example, are more likely to be infested by fleas or other bloodsucking parasites than are nests in small colonies. Experiments in which some burrows were fumigated to kill the parasites showed that these parasites lowered survivorship by as much as 50 percent in large colonies but not significantly in small ones. The swallows inspect and then select parasite-free nests in large colonies, they tend to build new nests rather than use old, infested ones. On balance, the advantages of colonial nesting clearly outweigh the disadvantages, given the many times at which colonial nesting has evolved independently among different groups of birds. Still lacking, however, is a general framework for testing different hypothesis for the evolution of coloniality.
1.The word “vicinity” in the passage is closet in meaning to
A.protection
B.region
C.population
D.resources
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.It is more advantageous for birds to choose central locations for their colonies rather than locations near the edges of their territory.
B.Compared to nests at the edges of colonies, centrally located nests are preferred for their safety from predators and therefore are more densely packed together.
C.Predators generally prefer the densely packed, central portion of nesting colonies, which can make this part of the colony more vulnerable to predators.
D.Birds nesting in colonies that are vulnerable to predators tend to prefer more densely packed nests to those less densely packed.
3.Paragraph 2 implies which of the following about yellow-rumped caciques?
A.They are comparatively unlikely to be harmed by the wasps that attack their predators.
B.They are able to protect their nests without using colonies.
C.Mixing active nests with abandoned nests is the least useful way of defending their nests.
D.Most of their predators are members of other bird species.
4.Paragraph 2 claims that yellow-rumped cacique colonies defend themselves from predators in all of the following ways EXCEPT:
A.They establish colonies in hart-to-reach places.
B.They physically attack invading predators.
C.They hide active nests among previously used ones.
D.They limit the size of their colonies so they are hard to find.
5.The phrase “contribute to” in the passage is closet in meaning to
A.learn from
B.depend upon
C.take advantage of
D.add to
6.According to paragraph 3, what advantage do birds gain by hatching all the colony’s eggs at the same time?
A.They are able to time the hatching of their chicks for when predators are not likely to be around.
B.Chicks hatch when food is abundant, which is generally only a few times a year.
C.Even if predators eliminate some of the newly hatched birds, many others will survive.
D.Weaker birds gain protection for their young by synchronizing their nesting behavior with that of the more dominant birds of the colony.
7.According to paragraph 3, cliff swallows closely observe other members of their colony in order to
A.follow successful birds to safe nesting sites
B.learn the location of local predators
C.place their eggs near other birds in the colony
D.find good sources of food
8.Which of the following is a probable effect of the fact mentioned in paragraph 4 that there are few available nesting locations near the Humboldt Current?
A.Seabirds compete with each other for a limited supply of food.
B.The number of seabirds at any one site is extremely large.
C.Some seabirds nest in conifer forests near locust outbreaks.
D.Colonies near the Humboldt Current contain small numbers of seabirds.
9.The word “exhaust” in the passage is closet in meaning to
A.use up
B.leave
C.avoid
D.hide
10.The phrase “On balance” in the passage is closet in meaning to
A.Nevertheless
B.Overall
C.Therefore
D.Periodically
11.In paragraph 5, why does the author discuss experiments in which some burrows were fumigated?
A.To demonstrate that parasites have a very negative effect on large colonies
B.To show that attacks by predators are a worse problem than the spread of parasites
C.To explain how swallows inspect nests for parasites
D.To prove that the benefits of colonial nesting outweigh the disadvantages
12.Which of the following questions is NOT answered by paragraph 5?
A.What causes colonies to have problems with their food supplies?
B.What are the disadvantages of colonial nesting?
C.What percentage of cliff swallow nests are infected by parasites?
D.How can one test the different hypotheses explaining the evolution of bird colonies?
13.Look at the four squares that indicate where the following sentence could be added to the passage.
The overall survival of the next generation is thus enhanced.
Where would the sentence best fit? Click on a square to add the sentence to the passage.
14.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 answer choices 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 question is worth 2 points.
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补充机经：
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