Monarch butterfly on a flower in Costa Rica.
© Ashleigh Whiffin
The Monarch is well known for its ability to travel great distances, and the migrations in North America are one of the greatest natural phenomena in the world. This butterfly is also very occasionally recorded from the British Isles, making it our largest and rarest migrant butterfly.
Large with a wingspan 95-100 mm
Caterpillars require Milkweed.
A native of the United States. It occurs wherever their foodplant grows; including farmland, gardens, parks and roadsides.
Monarchs fly up to 100 miles per day during their annual migration
Every autumn, millions of monarch butterflies migrate 3,000 miles from their breeding grounds in northeastern North America to spend the winter in the forests of southwestern Mexico. Each migration is by a new generation, so they cannot learn from others. Instead, they rely on their genes. Their antennae have a genetic clock that tells them when to migrate and to navigate they measure the sun's position on the horizon with their eyes.
The autumn migration of monarch butterflies is completed by just one super generation that can fly and live eight times longer than regular monarch butterflies. When spring arrives these monarchs reproduce and their offspring continue to complete their journey back to their summer breeding grounds.
The Monarch butterfly caterpillars feed solely on Milkweed plants - which is toxic to most animals. Monarchs have evolved to tolerate it and use it to their advantage, storing the toxins in their bodies and making themselves poisonous to predators.
Monarch butterflies must migrate because they cannot survive long cold northern winters. When they arrive in Mexico, they huddle together in on Oyamel fir trees. Each tree may be blanketed in tens of thousands of 'roosting' butterflies. They hang almost motionless in semi-dormancy, surviving the winter on their stored energy.
View this video of them overwintering en masse in Mexico.
Millions of Monarch butterflies migrate each year, but sadly their numbers are declining at a worrying rate. To protect them, the El Rosario Monarch Butterfly Reserve in Mexico was designated a UNESCO World Heritage site in 2008, as the concentration of overwintering butterflies there was deemed "a superlative natural phenomenon". However, recent assessments of this eastern population, have shown that during winter 2019-2020 the overwintering Monarchs occupied just 2.83 hectares of the Mexican forest, compared to 6.05 hectares the previous winter.
Climate change, pesticide use and habitat loss, both in the United States and in Mexico, all threaten their populations. To add to this, defending the roosting sites from illegal logging is very dangerous work. In January 2020, two butterfly activists were killed whilst trying to protect this forest.
To track their migration, hundreds of volunteers have been tagging the butterflies with small stickers. Recording where these tagged butterflies go has helped scientists learn more about their migratory routes, including the time and pace of the migration and any changes in geographical distribution.
Organisations like the Xerces Society are working with farmers, communities and roadside managers across North America, to restore and manage more habitat for Monarchs. Citizens can help by planting native milkweeds, cease using garden pesticides and by getting involved with citizen science projects. International support is needed to ensure that this incredible migration continues.
You can find out more about the migration of the Monarch butterflies in a new display in Animal Senses on Level 3 of the National Museum of Scotland.
Detail of the Monarch butterfly display in Animal Senses.