What looks like dead leaves are actually hundreds of Monarch butterflies clumping together. The butterflies that migrated to this grove this year are several generations removed from the ones who made the trip last year. None of these butterflies have ever been here, yet know instinctively just where to go.
Many butterflies were blown out of the trees by a heavy rainstorm the previous day, and can't fly until their wings dry. They sure don't seem to mind a "helping hand" to get off the wet ground.