Best Way to Work With Sessions Containing Different Topics

What is the easiest way to import selected photos in a session into different catalogs?

For example, I may have a session with photos of trees and birds. Then I'd want to move the photos of trees into the catalog where I save photos of trees.

Should I make copies of the folder containing the session and then edit/use each copy for the appropriate catalog? Or can I import only part of a session into a catalog?

I welcome your suggestions.

Thanks.