A Million Monarchs
The migration is so concentrated in the Sierra Madres that monarchs go through at exceedingly high rates. This is where the most spectacular sightings are possible. You might see a million monarchs one day—and none the next—in the very same place. Dr. Bill Calvert experienced a chance encounter one October.
"It was just absolutely phenomenal. Between 11 and 12 o'clock monarchs were passing overhead by the millions. They continued pouring over until about 1 o'clock. Seeing a million butterflies passing over is kind of magical."
Image by Jonathan Deuck Cumbres de Monterrey, Nuevo Leon October 17, 2004