From the Division of Wildlife website:

Spectacular! That is the only way I can describe the hundreds of butterflies that were fluttering over my head. One early fall evening just before sunset, waves of black & orange piled onto tree branches to rest overnight. This “monarch madness” did not occur in an exotic, tropical location. Rather, I witnessed this event at Killdeer Plains Wildlife Area in Wyandot County. Monarchs may be the most identifiable butterflies, even for those with little knowledge of their habits. Like most other insects, monarch butterflies cannot survive a long, cold winter. But unlike most other insects, monarchs deal with the oncoming winter by migrating to the California coast & mountains of Mexico. This migration is triggered as fall approaches & the days grow shorter. In Ohio, the monarch migration peaks in early to mid- September, with stragglers seen late into October. The monarchs we see passing through the state may have hatched as far away as northeastern United States and Canada.