You can configure Songbird to actively monitor (watch) your music folder and automatically update your library, when:
- Music tracks are added to the folder.
- Music tracks are removed from this particular folder.
To access these settings:
- Click on Tools > Options > Media Importer > Watch Folders
- Check "Automatically watch for changes in the following folder"
- And in the "Folder" field, enter the full path to your music folder.