关于鸟类飞行的起源，传统上提出了两种对立的假设：“地栖说”和“树栖说”。

Two opposing scenarios, the "arboreal" hypothesis and the "cursorial" hypothesis, have traditionally been put forward concerning the origins of bird flight.

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但是其它因素也会带来影响：栖地的流失、污染、疾病和入侵的物种。

But other factors have also had an impact: habitat loss, pollution, disease and invasive species.

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虽然森林以及湖泊的面积都有所增长，但是这与那些建起来的，工业的，人造的栖地的扩张比起来，不足称道。

Woodland, forests and lakes had grown, but these increases were dwarfed by the biggest habitat expansion, which was "constructed, industrial, artificial habitats".

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