are the main ingredient of birds' nest soup 是燕窝羹的主要成分

128. The work may be hazardous in the extreme,
     这项工作看起来极度危险
129. but the rewards are great.
     但却能得到丰厚的回报
130. The pure white nests of cave swiftlets
     洞金丝燕的纯白燕窝
131. are the main ingredient of birds' nest soup
     是燕窝羹的主要成分
132. and gram for gram are worth as much as silver.
     每克燕窝的价值堪比白银
133. As soon as its nest is removed a bird will immediately build another.
     燕窝被摘去后 金丝燕很快又会再吐一个
134. So, as long as this valuable harvest is properly controlled,
     所以 只要这种利润可观的 采摘行为得到适当控制

来自《行星地球 第一季 第四集 Planet Earth S01E04 洞穴》第 131 行