This year's population contains 57 million monarchs compared to a long-term average of 300 million and a peak of 1 billion. The clustering butterflies cover less than 3 acres of forest compared to a peak of 51 acres in 1996 and an average of 15 acres.

Half of this year's butterflies are residing in only one of the 12 traditional sites, the El Rosario sanctuary.

• Official Press Release

Still Dangerously Small
"The increase to 1.13 hectares is dangerously small, and lower than predicted," says Dr. Lincoln Brower.

When compared to populations over the past 21 years, monarch numbers continue to concern scientists as they watch for population recovery.

"Population levels are at their second lowest ever," notes monarch biologist Dr. Karen Oberhauser.

Upcoming Breeding Season Critical
Following two unfavorable summers that were partially responsible for the population crash in 2012, weather conditions during last summer's breeding season were ideal. If the population had encountered poor breeding conditions in summer 2014, scientists were concerned that the migration would reach its extinction threshold.

What does the target population size need to be to preserve the migration? Scientists simply don't know but a small population has added risks.

"With the population this small, chance events can have a huge impact," says Brower. For example, the winter storm in 2002 killed 85% of the population. With half of the population in one sanctuary the risks in Mexico are especially high this winter.