Bernd Heinrich - The Homing Instinct - Meaning and Mystery in Animal Migration

The picture that has now emerged from decades of study is that individual butterflies migrate all the way from Ontario to Mexico in the fall, arriving there at their overwintering sites in a torrent during October. They spend most of the winter in Mexico in a cooled low-energy state but soar around on warm days to drink water and replenish on nectar. In early spring, when their sex urge awakens, there is a mating orgy followed by a mass exodus. Most of the females mate before leaving, and their “compasses,” which were set to take them south in late fall, are now “reset” to take them in a northerly direction.

As the tide of butterflies advances northward, the females stop to lay their eggs on milkweed. Some of the butterflies from Mexico make it all the way to the north, and others (their offspring) that grow from the eggs laid along the way arrive later. Those of the first generation have slightly tattered wings when they arrive in the north, while those that arrive later have untattered wings. (However, not all monarch populations migrate, and not all that do, travel in the same directions as the populations of northeastern North America.)

One of the mysteries that puzzled Fred Urquhart was how the butterflies home. In Urquhart’s 1987 book on the monarch, he speculated that the butterflies perhaps use the Earth’s magnetic lines of force, although different populations of the butterfly migrate in different directions, so they could not all be orienting to it in the same way.

A potentially even more puzzling question is the ultimate (evolutionary) one of why these butterflies migrate in the first place. Urquhart simply suggested what he admitted was a “perhaps far-fetched” idea: that “twice each year it [Earth] passes through an area rich in some sort of radiation that could impinge upon animal life [that] might affect in some manner the cells of the body causing reproductive organs to abort in the fall and develop in the spring and initiate the migratory response.” This is an unlikely theory, though, mostly because it depends on a mechanism that is not adaptive in evolutionary terms. Instead, more current thinking about the adaptive reason why the phenomenon has evolved focuses on energy economy and maximization of resource use under the expected evolutionary constraints from the monarch’s having evolved in the tropics, meaning it was not able to survive northern winters. (Monarchs belong to the family Danaidae, an otherwise strictly tropical group.) Migration to the north in the spring opens up the milkweed crop over a major swath of North America as a food base for the larvae. In addition, the journey is probably not costly to the monarchs, either in terms of predation (since they are chemically protected from predation by poisons they sequester from their food plants) or in terms of energy costs, since their energy intake along the way more than makes up for the energy expended for travel. Indeed, unlike most birds that may deplete all their fat reserves on migration, these butterflies instead fatten up on their journey and may consist of about 50 percent body fat by the time they arrive in Mexico, where their overwintering fast begins.

Butterflies and moths experience tremendous selective pressure, and undoubtedly there are constant readjustments of survival strategies. Weather affects the populations, not only through flight activity and flight range as well as growth rates of larvae, but perhaps also indirectly by influencing virus infections. But Urquhart noted that each female monarch butterfly lays up to seven hundred eggs, and he calculated that the “biotic potential” — the number of individuals if there are no deaths — of one female after only four generations (that is, at the end of one summer) is 30,012,500,000 adults. Luckily for the planet, animals’ reproductive potentials are never naturally realized, for long. The limit is quickly reached when the population uses up its food base, in this case milkweed. In some years a virus decimates most of the monarch population over North America, but then several years later it rebounds. But the population cannot rebound from some things: in recent years there have been massive declines of the monarch population that cannot be reversed, because they are due to unnatural causes — the massive conversion of land to crops, and the introduction of genetically modified crops that tolerate herbicides, which have allowed the elimination of milkweed that formerly grew between rows of corn.

The flight performance of monarchs is spectacular, but like the hordes of cluster flies from the surrounding fields and woods that overwinter in my cabin, they are traveling to a specific place for overwintering where they have never been before. Such homing movements are diverse, but common. Robert D. Stevenson and William A. Haber of the University of Massachusetts, Boston, found a regular seasonal migration of about eighty percent (250 species) of butterflies living in the dry lowlands of the Pacific Slope of Costa Rica that migrate to wetter forests of the east. Distances traveled range from ten to a hundred kilometers.

In North America as well as in Europe, the cosmopolitan painted lady, Vanessa cardui, a mostly orange and black butterfly with white spots and pink and blue “eyes” on its under-wings, at times appears in large numbers and then is not seen again for years. Usually the individuals are seen crossing a road, and almost all will be heading in the same direction. The painted lady regularly migrates north from Mexico, from where it originates, after heavy rains in the deserts have created an abundance of food plants, primarily thistles. A friend told me of one migration while he was in Arizona when his windshield wipers “soon became useless” because of the huge numbers of painted ladies plastered onto them as he was driving. I see them regularly in Vermont and Maine, but seldom in large numbers (the summer of 2012 was one of the exceptions).

Red admiral butterfly larva, adult, and chrysalis. The larva makes a shelter for itself by pulling leaves together and holding them with silk, while then feeding on the leaf.

One of the butterflies that not only migrates as an adult but also hibernates in some parts of its range is the red admiral, Vanessa atalanta. It is (as are all butterflies!) beautifully colored. It sports a wide red stripe across each dark forewing ornamented with white spots, and its larvae feed on nettles. I wrote in my journal on May 11, 1985, near my home in Vermont: “In the afternoon from around 2:30 to 4:30 PM, as I was jogging along on an 18-mile circular loop I counted 512 red admiral, crossing the road in front of me. All but 5 of these were flying in a northeasterly direction. At 5:00 PM, after I was home, I take compass readings of butterflies flying over a plowed field where they funnel onto it through a valley. I can see them to take a bearing for at least 50 paces — 250 feet. All 22 that I observed flew in NE direction. At 6:00 PM activity almost stopped. The breeze is slight, from northwest.” In the summer of 2001 and again in the spring of 2010 I saw large numbers of red admirals. They fed on freshly opened apple blossoms, and later all the nettle plants in a neighbors’ sheep pasture had an abundance of their caterpillars.

Moth migrations are perhaps more spectacular than those of butterflies. Jason W. Chapman and colleagues report one recent ten-year study involving radar tracking of about one hundred thousand owlet (Noctuid) moths, primarily the silver Y moth, Autographa gamma, migrating south in the fall from northern Europe, and then north from the Mediterranean in the spring. Like the butterflies, these insects breed along their migration route. Also like the butterflies, the moths partially correct for crosswinds, to maintain specific directions. Most surprising perhaps is the moths’ windsurfing; they choose the most favorable wind currents corresponding to their respective spring or fall migratory directions. If the wind shifts about twenty degrees from the favorable direction, they adjust their flight to accommodate and maintain the correct direction. If the wind shifts ninety degrees, though, they stop and wait for a favorable wind. Millions of them fly together in the dark of night, and, like the monarchs’, their compass directions are likely tuned to the Earth’s magnetic fields. Some studies of radio-tagged green darner dragonflies, Anax junius, suggest that these insects also migrate hundreds to thousands of kilometers from north to south with those that return being a different generation.