译文解析：《飞鸟》——鸟类动物迁徙的原因

What gives the birds their overpowering urge to go? Most migrate to find food, rather than to escape the cold. When bird feeders became prevalent in northern American cities, the winter range of house finches and cardinals expanded, which suggests that even winter can't drive off some birds if they have enough to eat. Another reason is light. Birds may need extra daylight to rear their young or to protect themselves from night-stalking predators.

是何种强烈的欲望使鸟迁徒呢？大部分鸟类是为了觅食而迁徙，而不是为了躲避严寒。在美国北部一些城市，喂鸟的人越来越多时，家养鸣鸟及北美红雀的冬季分布区域便扩大了。这表明，如果一些鸟能获得足够的食物，即使在严冬他们也不会离开。日光是鸟类迁徙的另一个原因。鸟可能需要更多的光照养育雏鸟及保护自己免遭夜间觅食的食肉动物侵袭。

名词urge的意思是“强烈的欲望，冲动；推动力”，我们译成“强烈的欲望”。house finch和cardinal的意思是“家养鸣禽、北美红雀”。

条件从句的主句even winter can't drive off some birds的主语是winter，从句的主语是“鸟”，为了主从句的主语一致，把主句主语换成“他们”，原句的主语winter翻译成状语“在严冬”。night-stalking predator指“夜间寻食的食肉动物”。stalk指的是“潜步追踪，潜近猎物”。

But how do smaller birds that weigh mere ounces fly halfway round the world twice a year on nothing but two wings and a prayer? The way birds plot a course over thousand of miles of unknown country remains a mystery. Part of it is instinct. but it's also known that birds use topographic clues as natural street signs -- left at the mountains then straight down the river. They may also set their course by the position of the sun.

那些体重只有几盎司，身材娇小的鸟类是如何凭双翅及运气一年两次飞行过半个地球呢？鸟类测定飞往几千英里以外的未知国家的航线的方法仍然是个谜。其部分谜底是它们的本能。但人们也发现，鸟常以地形特征为路标——在山地向左，然后顺河流而下。它们也可能根据太阳的位置设定航程。

prayer的字典意思是“祷告”，这样翻译显然不通顺，从上下文看，小鸟飞行途中遇到很多困难，完成飞行也是一种运气，该词译成“运气”即可。plot a course译成“测定航线”。为了要表达的意思更清晰，Part of it is instinct中的it要译成名词“谜底”。Topographic意思是“地形的，地貌的”。