Species related to the Monarch Butterfly (Danaus plexippus) are found on every continent except Antarctica. But the Monarchs in North America are highly unusual. No butterflies in the world travel farther than they do. Every fourth or fifth generation of Monarchs migrates, covering distances as long as three thousand miles. And while other butterflies also migrate, the Monarch is apparently the only butterfly in the world that spends the winter in specific places--Monarchs from west of the Rocky Mountains migrate to roosting sites in southern California and those to the east of the Rockies go to sites in the Transvolcanic mountains of central Mexico. A Monarch ends up at the same sites where thousands, even millions of Monarchs spent the winter the previous year. An overwintering site can be as large as ten acres with an estimated 5 million to 6 million butterflies per acre, and within a site often the same tree is used year after year as a roosting place. Since the butterflies making this journey never made it before, it has long been unexplained how they go to the exact same place that their ancestors had gone before them. Recently, scientists at the University of Massachusetts School of Medicine made a discovery that contributes to an explanation of what makes this migration possible: they found that Monarchs have an internal clock that is set by the angle and intensity of the sun. Their ability to navigate to their wintering grounds is thought to be connected to their ability to detect changes in the sunlight they are exposed to. This does not explain how Monarchs are able to migrate to a precise location, but it does put a piece of the puzzle in place.
To begin to understand the Monarch more generally, it is perhaps first necessary to understand a key difference between those that emerge from their pupae during the summer and those that emerge at the end of summer or beginning of fall. The former live for only three to five weeks; the latter may live for eight or nine months, and it is they who make the long migration. The Monarchs born at different times of year have the same appearance: all have a one inch long body and a span of three inches across their wings which are orange with veins of black.
The similar appearance of all Monarchs is key to their survival, for its color is a warning that it may be toxic. Monarchs are found only where milkweed plants are found because their larvae feed only on plants in the milkweed family (Asclepiadaceae). Some milkweeds produce cardiac glycosides, and these are absorbed into the body by the Monarch caterpillar and stored there for the organism's entire life, making them poisonous to birds that would prey on them. A predator sees only the color and does not know whether a particular butterfly is poisonous or not. It simply avoids eating all Monarchs. The Monarch is so dependent on milkweed that it has been said that where one finds milkweed one finds the Monarch; where milkweed is more dense, so is the Monarch population. Indeed, the return migration in the spring is possible only when milkweed growth is far enough along that female Monarchs are able to deposit eggs on the underside of leaves.
After spending five months or so in their winter roosting areas, Monarchs begin heading back to where they came from. On their way the females lay eggs on milkweed plants along their flight path, having mated multiple times before leaving their roosts. It is unlikely that any monarchs manage to return to the place they started out from in the fall, but it is thought that the Monarchs that come from the first eggs laid in late March and early April make it to the northernmost breeding grounds--central and eastern Canada for those east of the Rockies, and Montana, Idaho, and Washington state for those west of the Rockies.
University of Kansas Entomology Program: Monarch Watch
This wide-ranging site has sections on Monarch biology and how to rear them and grow the plants, such as milkweed, that will attract them. Also here are pages on various research projects on Monarchs, including tagging programs, some of which students can participate in, as well as an excellent FAQ page and a number of photos.
University of Minnesota, Department of Ecology, Evolution, and Behavior: Monarch Lab
One of the intentions of this site from the University of Minnesota is to provide "solid background for understanding the ecology, behavior, and evolution" of Monarch Butterflies. It is also a good place to find results of recent studies, to read about student projects, and to get in touch with scientists, teachers, and students who are researching Monarchs. Among the highlights of this site are its pages on the life cycle of an individual Monarch and on the annual cycle of the several generations of Monarch that live in one year. Also provided is a page of useful links on Monarchs and butterflies in general.
Enchanted Learning: Monarch Butterfly
This useful page from Enchanted Learning is geared towards younger students and contains a number of useful illustrations as well as basic information on the life-cycle of the Monarch.
Journey North: Monarch Butterfly
This website about Monarch Butterfly migration is a part of Journey North: A Global Study in Wildlife Migration, an educational project presented by the Annenberg Foundation and the Corporation for Public Broadcasting. The Monarch page includes information on how to track Monarchs, migration updates, Monarch facts, and teaching resources. Through this website, students can even take part in a symbolic Monarch butterfly migration, an annual activity in which thousands of students in the US and Canada send homemade paper butterflies to Mexico in the fall, during the same period that the real Monarchs migrate there. The paper butterflies spend the winter "roosting" in Mexican classrooms until the Monarchs begin the return trip north, at which time Mexican students send the paper butterflies back with a note for the students who made them.
Digital Monarch Watch
This page from PathFinder Science has a variety of information on Monarch butterflies and their migration patterns. There are pages with background information, data that this team has gathered in their studies of the Monarch, and how that research is done.