Monarchs are especially noted for their yearly migration over long distances. In North America they make massive southward migrations starting in August until the first frost. There is a northward migration in the spring. The monarch is the only butterfly that migrates both north and south as the birds do regularly, but no individual makes the entire round trip. Female monarchs lay eggs for the next generation during these migrations.
By the end of October, the population east of the Rocky Mountains migrates to the sanctuaries of theMariposa Monarca Biosphere Reserve within the Trans-Mexican Volcanic Belt pine-oak forests in theMexican states of Michoacán and México. The western population overwinters in various sites in central coastal and southern California, United States, notably in Pacific Grove, Santa Cruz, and Grover Beach.
The length of these journeys exceeds the normal lifespan of most monarchs, which is less than two months for butterflies born in early summer. The last generation of the summer enters into a nonreproductive phase known as diapause, which may last seven months or more. During diapause, butterflies fly to one of many overwintering sites. The overwintering generation generally does not reproduce until it leaves the overwintering site sometime in February and March.
The overwintered population of those east of the Rockies may reach as far north as Texas and Oklahoma during the spring migration. The second, third and fourth generations return to their northern locations in the United States and Canada in the spring. How the species manages to return to the same overwintering spots over a gap of several generations is still a subject of research; the flight patterns appear to be inherited, based on a combination of the position of the sun in the sky and a time-compensated Sun compass that depends upon a circadian clock based in their antennae. New research has also shown these butterflies can use the earth’s magnetic field for orientation. The antennae contain cryptochrome, a photoreceptor protein sensitive to the violet-blue part of the spectrum. In presence of violet or blue light, it can function as a chemical compass, which tells the animal if it is aligned with the earth’s magnetic field, but it cannot tell the difference between magnetic north or south. The complete magnetic sense is present in a single antenna.
Monarch butterflies are one of the few insects which can cross the Atlantic. They are becoming more common in Bermuda due to increased usage ofmilkweed as an ornamental plant in flower gardens. Monarch butterflies born in Bermuda remain year round due to the island’s mild climate. A few monarchs turn up in the far southwest of Great Britain in years when the wind conditions are right, and have been seen as far east as Long Benningtonin Lincolnshire. In Australia, monarchs make limited migrations in cooler areas, but the blue tiger butterfly is better known in Australia for its lengthy migration. Monarchs can also be found in New Zealand. On the islands of Hawaii, no migrations have been noted.
Monarch butterflies are poisonous or distasteful to birds and mammals because of the presence of the cardiac glycosides contained in milkweed eaten by the larvae. The bright colors of larvae and adults are thought to function as warning colors. During hibernation, monarch butterflies sometimes suffer losses because hungry birds pick through them looking for the butterflies with the least amount of poison, but in the process kill those they reject.
One study examined wing colors of migrating monarchs using computer image analysis, and found migrants had darker orange (reddish-colored) wings than breeding monarchs.
Research also has overturned a prevailing theory that the migration patterns of the eastern and western populations are due to genetic reasons and that their genetic material was different. The American populations have been found to be distinct from the populations in New Zealand and Hawaii, but not from each other.