核心问题
Some birds will fly away to the south when the weather turns cold.
词典解答
情态助动词 will 有一个很重要的用法, 就是表示规律性、习惯性或典型性的动作 (You use will to say that something usually happens.)。
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When I get on a bus and I see an advert, I will read it. (EED) 每当我上了公共汽车，看到广告时，我总是要读一读。
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He'll talk for hours, if you let him. (CGEL) 如果你让他讲话，他总要讲好几个小时。
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... experts confirmed that a puma will not attack a human being unless it is cornered. (NCE, 3, L. 1) 专家们断定说美洲狮如果不受到围攻是不会伤人的。
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She will sit there for hours, waiting for her son to come home. (Zandvoort) 她常常在那里一连坐好几个小时，等待着儿子回来。
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Sometimes the cat will lie there all afternoon. (MD) 有时候那只猫整个下午都躺在那里。