Waiting for signs of a mass migration—a grand exodus that could occur any day—Estela Romero found the sanctuary at El Rosario still butterfly-filled. The monarchs can't stay in Mexico much longer, but they can't move north too quickly either. This week, explore how seasonal change affects monarchs, and the timing of their spring migration.

This Week's Update Includes:

• News: Watching and Waiting...
• Slideshow: Spring Migration: A Race Against Time
• Journal: Why Leave Quickly--But Not Too Quickly?
• Maps and Invitation: Get Ready to Track the Migration!

News: Watching and Waiting...

"Within the month of March—a relatively short time—the monarchs will vacate their winter haunt and begin the recolonization of North America," says Dr. Bill Calvert.

Estela Romero reports, "I decided to go to El Rosario Sanctuary this morning. I was astonished to see how full the sanctuary still is! It is too warm now for the monarchs to stay much longer. They must be leaving any time."

What signs did Estela notice when she visited the sanctuary yesterday? What clues do her pictures reveal?

• News from Angangueo by Estela Romero
• Noticias de Angangueo por Estela Romero

All Eyes on Texas!
Watch the migration map for the first signs of spring migration. It's about 600 miles from the sanctuaries in Mexico to the milkweed-rich regions of Texas. Early butterflies should appear any day.

"The leading edge of the spring migration typically crosses the Rio Grande around March 15th," said Texas insect-expert Mike Quinn.

Will the first monarchs have been reported by next week at this time? South winds are predicted from Thursday through Sunday. Let's see if the south wind delivers our first northbound butterflies of spring 2011!

"I was astonished to see how full the sanctuary still is!" wrote Estela Romero on Tuesday.