what stir the old folks out?

No, it's the solstice feast days that stir the old folks out. Or, to put it another way, the feast days encourage the old folks to go out.

So "cormorants" are the "old folks"?

A cormorant is a bird.

So the "folks" here only refers to "people"?

Folk means people, nothing else.

You must remember, ori, that this is poetic writing, not necessarily subject to a grammatic analysis. Cormorants are birds. Pine and fir are trees. A fane is a temple. So what the entire passage means is that the birds sit in the trees, as in a temple, and the holidays celebrated at the time of the solstice (probaly Winter Soltice in this context) encourage the old people to bestir themselves and to move.

Addendum: I'm not familiar with this poem, so I'm not sure but it's possible that the writer is refering to the birds -- cormorants -- as 'old folks' who've been sitting in the trees and now bestir themselves.

Put the rest of the poem up, please. It looks to me like you are trying to analyse a poor translation. If we have more text, this might become clearer.

The original text is an anti-copy version.

The whole context in the whole book is here:

http://books.google.com.hk/books?id=Sc-shy_ZOMkC&pg=PA377&dq=Fir+and+pine+by+the+fane+keep+cormorants&hl=zh-CN&sa=X&ei=QqzyTvKeBcyhiAf0_Y3OAQ&ved=0CDEQ6AEwAA

If too long to be clicked, copy it into your brower address bar and press enter.

http://books.google.com.hk/books?id=Sc-shy_ZOMkC&pg=PA377&dq=Fir+and+pine+by+the+fane+keep+cormorants&hl=zh-CN&sa=X&ei=QqzyTvKeBcyhiAf0_Y3OAQ&ved=0CDEQ6AEwAA#v=onepage&q=Fir%20and%20pine%20by%20the%20fane%20keep%20cormorants&f=false

I've read some of the book and comments now.
The meaning of the poem is quite obscure, but I'm sure "old folks" refers to people, and not to the birds.