|A monarch butterfly, Danaus plexippus.
I was in Cape May Point, New Jersey, on Saturday, September 18th, expecting to see migrating hawks. Many birds follow the Atlantic shoreline south during autumn migration. As New Jersey gradually narrows into a tip at its southern end, migrants end up concentrated at the “point” with the Delaware Bay in front of them -- across their path. They sometimes wait a day or two for a north wind to help them across the water. The place can fill up with eagles, hawks, and songbirds. It’s a great place for autumn bird watching.
But this year butterflies stole the show. Monarch butterflies also get funneled to New Jersey’s point. On Saturday there may have been a half a million big orange and black butterflies in the tiny town of Cape May. Butterflies filled the sky. They hung from the flowers and trees. It looked like the town had been transformed into a Walt Disney movie.
|Gathering in a roost for the night.
On Sunday morning, the wind was just right. The butterflies headed out in orange clouds across the bay. They passed by for hours. It was spectacular.
They are going to Mexico. As unlikely as it sounds, each tiny individual from the summer’s last generation flies the whole distance or dies trying. For some that is a distance of over 3000 miles. They can fly at about 12 miles per hour, for about 50 miles per day.
Once they get to Mexico, they spend the winter in a dormant state in chilly mountain refuges. The migrating generation lives about seven months, which is much longer than the few weeks typical for monarchs born in spring or summer. The migratory generation doesn’t finish developing sexually until the spring following migration. Then they rouse, mate, and start coming back north.
But they don’t have to fly all the way back. The return flight is generational. Individuals fly part of the distance, lay eggs on milkweed plants, and then die. The eggs hatch into caterpillars that eventually become adults that fly further north to lay eggs of their own. After a few generations, each traveling further north, the continent is repopulated all the way to southern Canada. Before you know it, a migratory generation is born and it’s time to migrate again.
Populations from eastern North America fly to Mexico. The western ones tend to end up in southern California. In cities all over the continent we can glance out of our office windows, even on high floors, and see the big orange butterflies passing by. The last of them will go by around Halloween; they have to beat the first frost.
There is more information about urban insects in my book, Field Guide to Urban Wildlife, which will be published by Stackpole Books in spring 2011.