网友的理解基本正确，我补充一点内容供网友参考。

It tires not, nor does it boast of its power; but(it省略) belongs to the air,travelling it may be six thousand miles to and from its northern nesting home, feeding its flown young as it flies, and slipping through a medium that seems to help its passage even when the wind is adverse.

【1】It tires not, nor does it boast of its power为分句(1)

【2】but(it省略) belongs to the air,travelling it may be six thousand miles to and from its northern nesting home, feeding its flown young as it flies, and slipping through a medium that seems to help its passage even when the wind is adverse.为分句(2)

备注：分句1和分句2构成转折关系的并列句

but(it省略) belongs to the air,travelling it may be six thousand miles to and from its northern nesting home, feeding its flown young as it flies, and slipping through a medium that seems to help its passage even when the wind is adverse.

【1】travelling it may be six thousand miles to and from its northern nesting home,

备注：travelling it may be....nesting time为倒装结构，还原后为it may be travelling six thousand miles to (its northern nesting home省略) and from(its northern nesting home), it为句子主语，may be travelling为句子谓语动词，six thousand miles为句子(距离)状语, to northern nesting home为句子(地点)状语，and为连词，from its northern nesting home为句子(地点)状语。

【2】feeding its flown young as it flies,

备注：以上为现在分词作状语，as it flies为as引导时间状语从句作现在分词短语(feeding its flown)的时间状语。

【3】and slipping through a medium that seems to help its passage even when the wind is adverse.

备注：以上为现在分词作状语，that seems to help its passage even when the wind is adverse.为关系代词that引导的定语从句修饰名词a medium，关系代词that在定语从句作主语，when the wind is adverse为定语从句时间状语，修饰定语从句谓语seem to help its passages.

 总结：2和3为由连词and连接2个并列现在分词短语作(方式/伴随)状语。修饰前面句子谓语:may be travelling six thousand miles to and from its northern nesting home。