74．The advantage of nesting on cliffs is the immunity it gives fromfoxes, which cannot scale the sheer rocks，and from ravens and other species of gulls, which have difficulty inlanding on narrow ledges to steal eggs.

(The advantage of nesting on cliffs is theimmunity it gives from…and from为句子主干)

在悬崖上筑巢的一大优势是可以避开不能攀登峭壁的狐狸和很难在窄峭的突出岩石上立足来偷鸟卵的渡鸦和其他鸥类。

分句1. Theadvantage of nesting on cliffs is the immunity it gives from foxes,

分句2. whichcannot scale the sheer rocks

分句3. and fromravens and other species of gulls,

分句4. whichhave difficulty in landing on narrow ledges to steal eggs.

1和3并列

1和2嵌套

3和4嵌套

1中省略which；3中省略it gives。2是1的定语从句；4是3的定语从句