"There are few Monarchs in El Rosario — only 10 trees at the most. Our impression is that they are arriving slowly and in small numbers," said Estela on November 16th.

Then finally, a massive arrival occurred:

"Yesterday (November 26), there was a very important arrival of Monarchs! Here in Angangueo, I could see large numbers flying at the entrance of the town, indeed heading towards the sanctuaries."

An Increase Predicted
The results of the annual population measurements — taken in late December — are typically released by March. We predict the population will have doubled from last year's record low, based on this fall's migration data and from overall impressions during breeding season.

A Lucky Break
Weather conditions during this year's breeding season were ideal, following the three bad years that led to the population crash. If this year's population does double — to 66 million butterflies — it would still be only 20% of the long term average.

How Many Needed?
If last year's record-low population had encountered poor breeding conditions, we don't know if the population could have recovered. What does the target population size need to be to preserve the migration? Scientists simply don't know the extinction threshold.