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Monarch Butterfly Migration Update: May 23, 2001

Today's Report Includes:

Monarch Population Expands and Spreads Across the North
When we left off last week, monarchs of the first generation had clearly emerged in force and were advancing northward. During the last week another 75 sightings have been reported, reflecting a surge in butterfly numbers. As today's migration map shows, the butterflies have now spread as far as 46 North.

Try This!
Graph the number of sightings per week so you can visualize the jump in population size, now that a new generation of butterflies has been born:


# sightings

May 16 - May 15


May 9 - May 15


May 2 - May 8


Apr 25 - May 1


Apr 18 - Apr 24


Apr 11 - Apr 17


Please Help: YOUR Monarch Sightings Are Important!
Even if early monarchs have already been reported from your state or province, we want to hear from YOU! The number of sightings we receive reflects how abundant monarchs are, so your observations continue to be important.

We'll continue to track the monarchs until they reach the end of the migration trail, sometime in mid-June.

This Week's Monarch Population Dynamics Meeting
Scientists and "citizen scientists" from the United States, Canada, Mexico and Australia gathered this week to address concerns about the apparent decline of the monarch population that migrates annually to Mexico from across eastern North America.

According to Drs. Karen Oberhauser and Orley Taylor who convened the meeting, "Recent reports on the degradation of the forest at the overwintering sites in Mexico, the possible impact of transgenic crops such as Bt corn on monarch populations, and the changes in agricultural practices that may affect the availability of milkweeds host plants, raise significant concerns about the potential impact of human activities on the dynamics of monarch populations."

The goal of the meeting was to identify gaps in knowledge about the factors that affect the size of the monarch population. Participants presented scientific papers and then addressed each stage of the monarch's annual cycle: The breeding season, fall migration, the overwintering season, and the spring migration. Groups summarized what is known about each stage of the cycle, and worked to identify priorities for research.

Of special interest to Journey North participants, the migration observations you are contributing are becoming very useful to scientists! Journey North migration data show where monarchs are located in time and space. Thus, the database can serve as an important indicator of the population's size each spring, and help to forecast the size of the next generation.

Says Dr. Lincoln Brower, "The database is so rich in terms of making it possible to address questions that were only speculative based on historical natural history information in the literature. For the first time, individual, qualitative records are being amassed into a quantitative picture."

Journey North program director Elizabeth Howard's presentation summarized clear and consistent patterns of remigration, based on an analysis of five years' data:
  1. A clear west to east progression of arrival across southern U.S. states, then up the East Coast ahead of the interior states at the same latitude, in apparent correlation with vegetation readiness
  2. Clear stalls and jumps associated with weather systems
  3. Expansion of the over-wintering population across eastern North America, typically to about 40N by first week in May.
  4. A noted increase in numbers when the first spring generation is on the wing.
  5. Differences in remigration patterns associated with spring temperatures in North America, and
  6. Differences in remigration patterns during years of high and low populations (based on winter population estimates at Mexican over-wintering sites).

What's more, two scientists are using Journey North data to support a new theory about monarch migration. In the presentation by Drs. David Gibo and Orley Taylor entitled "A General Theory to Explain the Movement of Migrating Monarchs Across Latitudes," they explained:

"We analyzed four years of Journey North first-sightings observations of Monarchs. The rate at which Monarch butterflies move north is correlated with the rate of northward movement of Altitude Angles of the sun, as measured at solar noon...One hypothesis on the adaptive value of the monarchs coordinating their northward migration by tracking a specific celestial cue is that this behavior brings them to their hostplant at the appropriate season."

Dr. Johannes Feddema, a geographer and meteorologist from the University of Kansas, has also incorporated Journey North data into a computer model. This kind of model will ultimately be used to simulate monarch population dynamics, and will serve as a critical indicator should populations fluctuate to dangerously low levels.

These are examples of scientific work that would not be possible without the valuable observations contributed by Journey North participants. Thank you for contributing your field observations to this permanent database! It was exciting to see its value, and the important role citizens can play in the scientific process.

Please report the FIRST MILKWEED to Emerge This Spring!

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The Next Monarch Migration Update Will Be Posted on June 1, 2001

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