But the desert is so big 不过这片沙漠很大

179. The bats left Mexico a few days earlier to escape the heat of summer
     蝙蝠数日前离开墨西哥以躲避酷暑
180. and are on their way north to the southern United States.
     正向北飞往美国南部
181. To get there, they have to cross the Sonoran desert.
     要想到达那里 它们必须穿越索诺兰沙漠
182. But the desert is so big
     不过这片沙漠很大
183. that for most of the year they would be unable to cross it.
     它们一年中大部分时间都无法穿越这里
184. Now, with the saguaro in bloom,
     现在 有了正在开花的弁庆柱
185. they can refuel on the way.
     它们便能在旅途中得到休整

来自《行星地球 第一季 第五集 Planet Earth S01E05 沙漠》第 182 行