Home » Gardening Information » Gardening for Wildlife
Butterfly Migration
by Pat and Clay Sutton
While walking the beachfront one morning at Cape May Point, New Jersey, we noticed a few Red Admirals and a Question Mark flitting by. Then another Red Admiral zipped by, followed by others. Scanning down the beach and out over the Atlantic Ocean and Delaware Bay, we saw dozens of butterflies streaming in off the water. By noon, as the southerly breezes built, there were hundreds and hundreds passing, and as we did a 360-degree binocular scan, we saw nearly a hundred butterflies a minute, all surging north. This unusually huge migration on May 22, 2001, went on until dark, with legions of leps washing over the New Jersey shoreline.
Migration is defined as any seasonal movement between two areas. It is found throughout the animal kingdom and is quite common among insects such as butterflies and dragonflies. Many species of butterflies are highly migratory. Due to the butterflies' inability to fight strong winds, their migrations are usually not as concentrated as those of birds, but on rare occasions an amazing spectacle every bit as dramatic as the annual migrations of birds can be observed.
Monarchs resting for the night on an eastern red cedar, Juniperus virginiana, in Cape May, New Jersey, during their fall migration to their Mexican overwintering grounds.
However, not all butterflies migrate; many are sedentary creatures. While afield you may see a Tiger Swallowtail disappear down a forest road, but it will probably not travel far beyond the neighborhood, town, or nearby forest. Some species of butterflies do not even wander. Generation after generation of Bog Coppers may be found only in and around a small bog where cranberry (Vaccinium macrocarpon), their only caterpillar food plant, grows.
Other butterflies, though, are short-, medium-, or even long-distance migrants, and these include some well-known favorites, such as Mourning Cloak, Painted Lady, American Lady, Red Admiral, and Common Buckeye. Over most of the country, some Mourning Cloaks, tortoiseshells, Question Marks, and various commas overwinter as adults, but local populations are almost always supplemented by fresh arrivals from the south each spring. American Ladies, Painted Ladies, Red Admirals, Common Buckeyes, and Variegated Fritillaries are cold-sensitive and cannot normally winter in northern areas due to the extreme winter temperatures. All arrive fresh in spring, migrating in from the south.
In fall, as temperatures drop and daylight hours shorten, descendants of these butterflies begin their migration south. Along the East Coast, steady streams of Mourning Cloaks, American Ladies, Painted Ladies, Red Admirals, and Common Buckeyes are seen heading south along with Monarchs, usually on gentle north or northwest tailwinds following a September or October cold front. Some of these cold-sensitive butterflies winter in the Southeast and Gulf states. Others, like the Painted Lady, are going to the southwestern states, northern Mexico, or even farther. Their offspring will then repopulate all the way north into Canada the following year.
Migrating Monarchs
The Monarch is the long-distance champion of butterfly migration, performing the longest migration of any insect in the world that we know. Each fall many millions of Monarchs, sometimes 250 million or more, empty out of the United States and southern Canada, east of the Rocky Mountains, and journey south to overwintering roosts in the mountain fir forests west of Mexico City. Monarchs from the northern edge of their summer range in Canada may migrate 3,000 miles to reach these winter roosts.
Monarchs are strong fliers, flying up to 20 miles an hour and sometimes at heights of over 10,000 feet. Monarch tagging has proved that they can travel at least 80 miles in a day. In the East, amazing concentrations of migrating Monarchs can be witnessed in September and October. Sometimes thousands of Monarchs can be seen roosting in eastern red cedars or clinging to stands of nectar-rich seaside goldenrod (Solidago sempervirens), which grows wild on coastal dunes. At this time blooming patches of Mexican sunflower (Tithonia rotundifolia), sedum, and New England aster (Aster novae-angliae) in backyard gardens might also be covered with migrant Monarchs. Numbers can be almost unbelievable. In 1999, the Cape May Bird Observatory's Waterbird Migration Count at Avalon, New Jersey, on the Atlantic coast 16 miles north of Cape May, counted 85,659 Monarchs migrating by between September 22 and November 21, over half of them on one day in early October.
An often-asked question is, "Why aren't similar Monarch migrations seen in spring?" In spring, the migration is protracted, and it takes successive generations to repopulate the United States and Canada. Many short-lived generations move north in an incremental manner, laying eggs and dying, with their offspring moving farther north and eventually repopulating the summer range. In fall, individual Monarchs make the entire journey south and then overwinter.
A far smaller population of Monarchs (about 5 million) migrates from western states and British Columbia to overwintering sites along the California coast. There they congregate in groves of eucalyptus and Monterey pine (Pinus radiata) from just north of San Francisco to just south of Los Angeles. Recent research shows that some western Monarchs cross the Rocky Mountains and go to the Mexican winter roost sites.
Emigrating Butterflies
While a number of butterfly species migrate "both ways," north in the spring and south in the fall, the movement of many butterflies is one way only. In summer (particularly in late summer) and fall, many southern butterflies perform an irregular one-way emigration north, leaving their natal area and ending up far to the north of their normal range. This is an exciting time for butterfly watchers. In late summer and autumn butterfly watchers in Cape May eagerly await such southern species as the Cloudless Sulphur, Little Yellow, Sleepy Orange, Clouded Skipper, Fiery Skipper, Sachem, and even such rarities as the Gulf Fritillary, Long-Tailed Skipper, Eufala Skipper, and Ocola Skipper. These are all species whose normal range is across the southern United States and further south into Mexico. In the West, California Tortoiseshells and American Snouts are particularly well known for their massive irruptions, at times involving many millions of butterflies.
A Painted Lady makes a pit stop during migration and sips nectar from New England aster, Aster novae-angliae, in fall.
Little understood by scientists, butterfly emigrations usually follow a good breeding season in the south. Adult butterflies may be reacting to overpopulation and the resulting depletion of food sources, both in caterpillar food plants and nectar plants. Emigrants may raise one or more broods far to the north of their usual range, but there is no return south for these species, and all eventually die, doomed by falling temperatures and the onset of winter. Nevertheless, this is the vehicle by which species expand their range, and in this age of global warming, a number of emigrants, such as Sachem and Little Yellow, are continually pushing the limits, extending their ranges farther north.
Some northern species irrupt as well, and New York-area butterfly watchers sometimes see southward movements of Compton Tortoiseshell, Milbert's Tortoiseshell, and even rarities such as Gray Comma. Their behavior may be little understood, but strays or vagrants far from their normal range create some of the most exciting moments in butterfly watching. We remember one balmy fall day when a Gulf Fritillary, found in a nearby butterfly garden, literally emptied the Cape May Hawkwatch Platform as avid visitors stampeded to see one of the first Cape May records for this butterfly in many years.
Understanding the complexities of butterfly migration is a major aspect of butterfly watching, and attracting migrants is one of the greatest pleasures of butterfly gardening. They may be here today and gone tomorrow, but a healthy and diverse butterfly garden can be an important pit stop where migrating butterflies can refuel for their continuing journey. This is particularly true for Monarchs and other "two-way" migrants such as Red Admiral and Painted Lady, but important even for emigrants. One early autumn day, our garden's regular "local" butterfly residents shared the asters and coneflowers (Echinacea) with such visitors as Cloudless Sulphur, Fiery Skipper, Ocola Skipper, and, amazingly, a robust Brazilian Skipper! Such is the joy of butterfly watching and gardening. Plant it and the migrants will come!
Help to Preserve an Endangered Phenomenon
The widely loved Monarch is not an endangered species; it has been known to rebound quickly after years of drought or snowstorms in its Mexican overwintering grounds. The continuation of its yearly migration from Canada to Mexico is far more uncertain. Scientists are so concerned about the future of Monarch migration that they have dubbed it an "endangered phenomenon." Deforestation in central Mexico degrades Monarch overwintering sites by opening up the forest canopy and creating an unfavorable microclimate for the clustering butterflies. Across the border on the U.S. side, milkweed is often targeted as a weed in agricultural areas, which means the critical food source for Monarch caterpillars is in jeopardy along the entire migration route.
Nectar-rich fall-blooming flowers, such as goldenrod, Solidago, provide migrating Monarchs with much-needed nutrients to sustain them during their long journey to Mexico
While it is vitally important to support conservation efforts in Mexico, gardeners along the migration route can help Monarchs by growing milkweeds. The plants are sought for egg laying by northbound females in spring and by ensuing generations of Monarchs through the summer, while milkweed nectar attracts many species of butterflies, including Monarchs.
As the last generation of Monarchs for the year makes its arduous journey south in the fall, individuals can cover long distances in a day. They need an energy boost from nectar-rich flowers that bloom late in the season, such as joe-pye weed (Eupatorium), asters, goldenrods (Solidago), and sedums. At night, they look for safe roosting spots in the branches of trees and shrubs. Join with your neighbors to create safe-travel wildlife corridors of pesticide-free milkweeds, nectar plants, hedgerows, and trees.
Finally, support the efforts of Monarch Watch, Journey North, and other organizations, to fund research, preservation, tagging, and educational outreach.
Claire Hagen Dole
Pat and Clay Sutton are co-authors of How to Spot Butterflies, How to Spot an Owl, and How to Spot Hawks and Eagles. Pat is program director for New Jersey Audubon Society's Cape May Bird Observatory and a board member of the North American Butterfly Association. Clay is a wildlife biologist and lecturer. They live near Cape May, New Jersey.
Top and bottom photos: Jane Ruffin; middle photo: David Cavagnaro