Imagine yourself walking in the woods with children. One child is saying, "there is a big hole in that tree's trunk." You answer, "perhaps a/some bird lives in there."
Would you use a or some? Please, elaborate.
I would probably use a in this sentence, but you could use either.
One of the meanings of some is:
So to refer to "some bird" living in a tree trunk simply means it is an unknown or unspecified bird, if it does in fact exist. But the indefinite article accomplishes the same task, and does so with a single letter, so I would prefer it in this case.