Just as you can import references from external files into your scene, you can also export objects in your scene to external files and keep them as references.

To export the willow tree

1. Open the Outliner (Window > Outliner).
2. Select willow1MeshGroup.
3. Select File > Export Selection > .

   The Export Selection Options window appears.

4. Select Edit > Reset Settings.
5. Turn on Keep only a reference.
6. Click Export Selection.

   The file browser appears.

7. Enter the name tree and click Export Selection.

   If you check the Reference Editor, you can see a new entry called treeRN tree.ma. The main scene is now referencing the tree from an external file, just like your zombies.