How do scientists study migration?

The amazing phenomenon of monarch migration has fascinated scientists for decades. Many methods have been employed in the attempt to unravel the mystery of monarch migration, including tagging programs, monitoring programs, and more technical chemical methods. Many of these programs have involved citizens of Canada, the United States, and Mexico in a cooperative effort to learn more about this remarkable journey.

Tagging

Dr. Fred Urquhart of the University of Toronto began a tagging program in the 1930’s. After thousands of tagged butterflies and several decades of work, the overwintering roosts in the mountains of central Mexico were finally discovered in 1975. Although local residents had known about the roosts for generations, no one from outside the area had reported them. This collaborative effort continues today as we attempt to learn more about this migratory phenomenon.

Dr. Chip Taylor of the University of Kansas has continued the study of monarch migration through a different tagging program called Monarch Watch. Started in 1991, Monarch Watch is a collaborative network of hundreds of thousands of students, teachers, volunteers and researchers dedicated to the study of the monarch butterfly. These participants tag tens of thousands of monarchs each year throughout Canada and the United States. Through the recovery of tagged monarchs, we have learned a great deal about the routes monarchs take and how fast they move.

Monitoring Programs

Other organizations have formed with the goal of monitoring monarch migration in a way that doesn’t require catching and tagging butterflies. Journey North is one such organization. It was established in 1991 to with two goals in mind: to improve science and math education and to study several species of migratory animals. Journey North involves school children from every state in the United States and 7 Canadian provinces. These students report their first sightings of monarch butterflies every spring. Through these reports, we can learn about when and where monarchs travel as they migrate north in the spring.

Texas Monarch Watch is another organization that enlists citizens to collect data on monarch migration. Dr. Bill Calvert, of Texas Parks and Wildlife, organized this program in an effort to understand the movement of monarchs through Texas during their fall migration to Mexico and their spring migration northward. Volunteers call in reports of monarch sightings, providing information about where, when, and how many monarchs they have seen. This information helps us learn about major flyways through Texas and, by comparing sightings over several years with weather patterns, we can learn about how weather influences monarch migration.

Other monitoring programs include the Monarch Monitoring Project run through the Cape May Bird Observatory Center for Research and Education and the Western Monarch Migration Project run by Dan Hillburn of the Oregon Department of Agriculture.

Stable Isotopes

Stable isotopes are different versions of regular atoms that have different masses. For example, the common isotope of hydrogen has one proton and one electron. The hydrogen isotope called deuterium also has a neutron, and is almost twice as heavy as the common hydrogen isotope. Scientists can use these differences between atoms of the same element to identify the "signature" of the breeding grounds from which a monarch originated. They can do this because different parts of the world have different amounts of the various isotopes of a particular element. Rainfall is the likely cause of the difference in hydrogen isotopes, but other weather patterns and geology can cause variation in hydrogen and other isotopes. When plants take up water, they obtain a isotope pattern that reflects that of their geographical region. When monarch larvae eat milkweed plants, they "inherit" this isotope pattern as well. Scientists can first identify the isotope "signature" of various geographical regions, then determine the isotope pattern of a monarch to roughly determine its origin. (For a more detailed description of how this works, visit the "Why Files" page on stable isotopes and monarch migration.)

Leonard Wassenaar and Keith Hobson of Environment Canada, Saskatoon, Canada, conducted a field study in which they collected monarch butterflies from the 13 known overwintering sites in Mexico and analyzed each monarch to determine its isotope pattern. They then matched these isotopic patterns with "signatures" they had identified previously. The found that about half of the 597 monarchs collected originated in the Midwestern corn and soybean belt.

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