Save the Monarch

Overwintering Monarchs

After a phenomenal two month long migration from the United States and southern Canada, beginning in August, the North American monarch butterfly reaches Mexico, where it spends the winter months.

The monarchs cluster in Mexico's rare oyamel fir forests, occasionally taking shelter in pines and other trees. The oyamel trees provide much needed refuge and protect the butterflies from extreme temperatures, rain, snow and predators. As temperatures drop over the winter, monarch movement decreases, and the butterflies form large, dense clusters on oyamel branches, coloring the forest orange.

By mid-December, monarchs have settled into their overwintering homes. With colder temperatures, monarchs gather in several predictable areas, with little movement, This is when the overwintering count takes place. The monarch population is estimated by the total area they occupy in the overwintering grounds, and has been conducted by the World Wildlife Fund and the Mexican National Commission of Protected Natural Areas since the winter of 2004-05.

Mexico established the Monarch Butterfly Biosphere Reserve in 1980 to protect the monarch's mountainous home. Just over 60 miles from Mexico City, the 138,000 acre reserve is sectioned off into several sanctuaries that provide winter refuge to the millions of monarchs who migrate to Mexico each fall. From roughly late October through February, monarchs live in the forested mountains of Mexico, where temperatures are mild enough for survival. This habitat is only found on 12 mountaintops on the planet, and is essential to the persistence of the monarch and its migration.

Chart showing total area occupied by monarch colonies at overwintering sites in Mexico. Winter season 1993 - 2017.  The same data is also provided in table form.

Data from the 1993-2003 was collected by personnel of the Monarch Butterfly Biosphere Reserve of the National Commission for Natural Protected Areas in Mexico. Beginning in 2004 data was collected by the World Wildlife Fund-Telcel Alliance in coordination with the Commission. Graph courtesy of World Wildlife Fund.

Area of forest occupied by monarch butterflies hibernating in Mexico
Year (data collected during December) Total area (acres)
1993 15.39
1994 19.3
1995 31.16
1996 44.95
1997 14.26
1998 13.74
1999 22.16
2000 9.46
2001 23.13
2002 18.63
2003 27.48
2004 5.41
2005 14.6
2006 16.98
2007 11.39
2008 12.5
2009 4.47
2010 9.93
2011 7.14
2012 2.94
2013 1.66
2014 2.79
2015 9
2016 7.19
2017 6.12

Current status of the monarch population

The 2017-2018 monarch butterfly population estimates reflect about a 15% decrease in the area occupied by monarchs in the overwintering habitat since last year. Overwintering monarch butterflies occupied approximately 6 acres of habitat in Mexico this year compared to last year's estimate of 7 acres. Every conservation action is needed to restore the eastern population of monarchs.

To provide some context, in the winter of 2013-14, experts reported the lowest monarch population on record with an occupied 1.66 acres of overwintering habitat. In 1996-97, monarch populations peaked with estimates reporting more than one billion monarchs occupying 44.5 acres of habitat.

You can help!

You can help monarchs as they prepare to migrate between Mexico, the U.S. And Canada each year by planting native milkweed and wildflowers. Avoid tropical milkweed and delay mowing during times of peak monarch activity in your area. Everyone and every little bit of habitat can help. The more monarchs we have, the better they can withstand extreme weather and climate events.