Naina B. answered 12/14/13
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Saori,
The point of the paragraph is the onset of the activity by birds in response to the weather; in some species the length of the day triggers the hormonal change. While weather might change the day but that is not the main point and therefore, A is incorrect.
B and C are not the direct outcome of the change of the weather and therefore, those are incorrect.
Migration of birds is the direct outcome of the weather, it is included in statement D and therefore, D is the correct answer.
Hope it helps.