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题目材料：
Kacelnik proposed that birds sing more at dawn and dusk because low light levels make feeding relatively unprofitable. Colder temperatures also make prey less active and thus harder to find. So why do birds wake up before they can feed, and why don't they wake up much earlier to lengthen the dawn chorus? It may be that other birds are unresponsive until only shortly before foraging becomes possible. Birds could listen to mates or neighbors singing overnight, but much social behavior contingent on that signal might plausibly occur only when there is some light. Such social interactions like chasing intruders or mating could be performed effectively at light levels at which foraging is still inefficient.
以上解析由 考满分老师提供。
