Monarch Migration Update: Sept. 15, 2011
Please Report
Your Sightings!

Hundreds of people from Canada to Mexico reported migrating monarchs last week. When the butterflies came down to rest, some lucky people discovered an overnight roost: "We had hundreds of monarchs staying last night in the trees out back. What an amazing sight!" This week, learn about the overnight roost and the key role citizen scientists play in studying them.

This Week's Update Includes:


Image of the Week

Overnight roost of monarch butterflies during fall migrationImage: Pat Swerkstrom

The Overnight Roost
Why so hard to study?

News: Migrating Monarchs from Canada to Mexico

Peak Migration Along the Great Lakes
Monarchs were on the move this week along the Great Lakes' shorelines. People on the north shore of Lake Ontario scored the season's highest count so far:

September 10: "While swimming at North Beach Provincial Park we counted 269 monarchs in 1 hour. The butterflies were presumably heading west along the north shore of Lake Ontario."

September 11: "I observed approximately 60 butterflies per hour moving southwest along Ipperwash Beach. The butterflies leave Canada and enter Michigan about 30 miles south west of here."

  • Do the math! How many monarchs per minute did each observer see?

Roosts: Fewer, Smaller, and Short-lived
So far this fall, only 43 roosts have been reported compared to 156 reports last year at this time. (See chart.) This year's roosts have also been small. Most have had only a few hundred butterflies and the largest contained about 1,500. Last year at this time, the largest roost had 10,000 monarchs (See photo.) This fall's roosts have also been short-lived. Most have lasted only a day or two. What might these observations mean?

  • Are there fewer butterflies this year?
  • Have fewer roosts formed because overnight temperatures have been warmer this year?
  • Did the butterflies leave early—or are they yet to come?

What Tales do Maps Tell?
Maps help us visualize data. We can see things we might not otherwise notice. Compare these two fall maps:

The Map of All Monarch Sightings
The purpose of the map of all monarch sightings is to show monarch distribution. According to the data reported so far, monarchs are present in ___ Canadian provinces, ___ U.S. states, and ___ Mexican states. How do you predict this map will change in the weeks ahead?

The Map of Overnight Roosts
The map of overnight roosts reveals information about the distribution and abundance of migrating monarchs. Roosts are places with large numbers of monarchs. They show us where migrating monarchs are most abundant. In the weeks ahead, the roost map will reveal the main migration pathway to Mexico.

  • According to the data reported so far, where have migrating monarchs been most abundant?
  • Where do you predict roosts will be reported next?

Monarch Butterflies at roost site in Mexico.
Last Year on Lake Ontario


Graph: Number of roosts reported as of September 14th each year
Number of Roosts


Seeing Monarchs?
Report Regularly!

Tell us when and where monarchs are present.

Please report at least once a week.



Monarch Butterflies at roost site in Mexico.

Scientific Findings
Fall migration flyways revealed by citizen scientists.


Slideshow: Why do monarchs form roosts during fall migration?

Scientists have many questions about where, how, and why monarchs form roosts during fall migration. Find out what is known about overnight roosts and explore the many mysteries that remain. What can we learn about fall migration by studying overnight roosts?


The Annual Cycle of the Monarch Butterfly


The Migration: Maps and Journal Page
Monarch Butterfly Migration Map: All Sightings, Fall 2011 Monarch Butterfly Migration Map: Fall Roosts, Fall 2011 Migration Questions: Week 2

All Sightings

(map/ sightings)

Fall Roosts


For Your Journal
This Week's Question

Seeing Monarchs? Please let us know!

The next Monarch Migration Update will be posted September 22, 2011.