We have a butterfly tree in our yard! Actually, we have two of them. We have some trees where monarchs are roosting in preparation for their migration.
When they hang from the stems or the leaves they are almost invisible. If another monarch happens by they show the dorsal portions of their wings–bright orange against the foliage.
I watched while individual butterflies would fly in and find the group. How do they do that? Is there some kind of chemical signal that they use? Other butterflies did not move until the new butterfly was close.