阅读里面句子的语法问题，谢谢大家

fannaoderUID: 3649030

More probable is bird transport, either externally, by accidental attachment of the seeds to feathers, or internally, by the swallowing of fruit and subsequent excretion of the seeds.

这句是og里的阅读里的句子。我想问的是，either externally这里为什么是副词。。externally修饰的是啥。。

谢谢哦。。