How Do Scientists Study Migration?
Knowing When To Leave
Where Do They Go?
Finding Overwintering Sites
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.
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
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.
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 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.
Continue to: References