On the weekend of April 14 and 15, I began to receive emails and calls about strange butterflies filling readers' yards. These unexpected insects began to appear a few days later in my yard, as well, fluttering back and forth between our junipers and my wife's favorite flowering viburnums.
They were red admiral butterflies. These erratic fliers have rich orange stripes against a brown background that is also broken by a few white dots near the wing ends. Their underside has just one of those orange stripes against a mixed pattern that even includes a few blue markings. Like monarch butterflies, these were butterfly migrants. They probably spent last winter in Texas.
I have never seen this many red admirals here. I find them more often when I visit my in-laws in Alabama. Thus, I believe this invasion was what is sometimes called an overflight. They started north and, encouraged by that hot March weather, flew farther than they usually do.
Several southern bird species occasionally occur here in early spring for the same reason. This year, for example, we have had a yellow-throated warbler and a cattle egret visiting the area. (That yellow-throated warbler is not to be confused with the yellow-throat, another warbler that is a summer resident in the marshes of our region.) In some years, white-eyed vireos similarly reach our latitude briefly in spring.
A few red admirals overwinter far north of Texas, hibernating in protected areas, but most of those butterflies do not make it through cold winters and the species population has to be replaced by migrants.
This past winter was surely a boon to our regular stay-over butterfly species like the mourning cloak and Compton tortoise-shell. A few years ago, I found a mourning cloak hibernating under a rock in our back yard.
Those red admirals will soon mate, lay their eggs and die. By June and July, they will have gone through their caterpillar and chrysalis stages and a new group of butterflies will emerge. This will be repeated and a final bloom will occur in September, with most of those flying south.
There are many unanswered and even more only partially answered questions about migration. If you venture out and listen, on many nights you will also hear chirps from the huge flocks of birds flying north.
One hypothesis is generally accepted: It was the retreat of the most recent of the massive glaciers that covered virtually all of Canada and our area, as well, that led to the migration of many species.
The ice that had reached the hard-to-believe height of two miles finally melted away from our region about 11,000 years ago. This ended (we hope) a series of glacial episodes that had continued for hundreds of millions of years.
Clearly that ice sheet supported very little wildlife and, in particular, virtually no birds or butterflies. And the associated cold climate made life difficult for them far to the south of the glacial limit, which in our area was generally along the Pennsylvania border.
But as the ice retreated, a new environment was created. The area became green again, creating attractive habitat for other wildlife. Foragers moved in to feed on the rich botany of the newly created region, and predators followed. Among these pioneers were certainly birds and butterflies.
Many early colonizers were punished for their audacity. The ice may have retreated, but annual cycles of cold and warm weather continued. During some harsh winters, a large proportion of those colonizers died. But a few retreated, just as the entire population had retreated in the face of the advancing glaciers. And this became their annual cycle of migration.
We see this process played out today as southern species move northward. One in particular is the Carolina wren. Its cheery "teakettle, teakettle, teakettle" song was common here for a few years, but then harsh winters reduced their population. I hope our mild 2011-2012 winter will encourage more to come and stay.