26 September 2021
In mid-to-late September monarch butterflies (Danaus plexippus) pass through western Pennsylvania on the way to their winter home in the Sierra Madre Mountains of Michoacán, Mexico. Last week I saw quite a few flying southwest, one at a time.
Each butterfly travels alone, migrating 50-100 miles a day and resting at night. By the time the eastern population has reached Texas their paths converge.
They look for good roosting habitat and end up together, often in pine trees, to wait out the night or bad weather.
When flight conditions suddenly improve they are spurred to leave. They all depart at once. That’s what happened on 21 September 2021 in west-central Texas after a cold front passed through.
Watch waves of monarchs light up the radar in this Facebook post at Garden Naturally Group.
(photos from Wikimedia Commons, map from US Forest Service; click on the captions to see the originals)