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The need for kin recognition in bank swallows arises only toward the end of the nestling period, because adults never mistakenly visit the wrong burrow to provision chicks. However, when offspring are about sixteen or seventeen days old, and before they fledge, they start to move between nests. Females commonly evict foreign chicks from their burrows, but males are prone to misfeeding strangers. Experiments suggest that females are able to recognize their own young only when the young reach sixteen or seventeen days of age, at which point offspring start making a distinctive two-note call. In cliff swallows visual cues might be used in addition to vocal signatures for identification of offspring, as faces of cliff swallow nestlings become distinctive at about thirteen days. Why does the cliff swallow need two recognition modalities when the bank swallow gets by with only one? Family-specific calls of bank swallows are localized to individual nests because the sandy banks in which they nest absorb sound so effectively that birds are unlikely to hear the calls of neighboring families. Thus the risk of recognition error is remote. The mudcups in which cliff swallows nest are more reverberant, however, so nestling calls can easily be overheard by neighbors.

Which of the following, if true, would also explain why bank swallows need only one recognition modality?

A.Bank swallow chicks have a shorter nestling period than do cliff swallow chicks.
B.Bank swallows have smaller clutches of eggs than do cliff swallows.
C.Bank swallow nests are much farther apart than are cliff swallow nests.
D.Bank swallow nests are less prone to predation than are cliff swallow nests.
E.Bank swallow chicks tend to be older when they begin moving from nest to nest.

C显示答案

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