请帮忙分析一个句子

请帮忙分析下面的句子，尤其是but这句话，我找不到主干呀

If the mother goose is
not present during this time, but an object resembling
her in important features is, young goslings may imprint on it instead.

If the mother goose is not present during this time, but an object resembling her in important features is, young goslings may imprint on it instead.

三句加起来是一个完整的条件句，前两句是并列条件句，后面是主句。but是并列连词，表转折。

是说：如果mother goose在这段时间没有出现，而另一种与她类似的东西出现了，那么young goslings就会对这种（不是它妈妈）的东西imprint.