Birthdays on the monarchs’ migratory flight

Each year, millions of monarch butterflies migrate from Mexico to southern Canada, but until recently, it was not known if the butterflies flew straight here or were born along the way.

By sampling genetic markers and wing wear patterns on monarch butterflies, researchers from the University of Guelph and Environment Canada have found that only about 10 per cent of monarchs that migrate to Canada make the 3,000 kilometre trip straight from central Mexico.

Most monarch butterflies migrating to Canada are born in the central United States. Photo courtesy of Andrea_44 on flickr.

“It wasn’t clear where these individuals were born and how long they lived,” says lead researcher, Prof. Ryan Norris from the university’s Dept. of Integrative Biology.

After studying monarchs in 44 sites across Ontario and the northern states, the researchers found that 90 per cent of the monarchs to reach Canada in late spring were born in the central United States.

But this migration is becoming increasingly threatened as urban sprawl and environmental changes reduce the availability of milkweed, a plant where monarchs lay their eggs.

Norris says that knowing where the butterflies stop to breed may allow scientists to forecast how environmental change and habitat loss will impact the yearly migration.

“Linking these periods of the breeding cycle provides us key information for conservation,” says Norris.

And milkweed is not just important as a spot for monarchs to lay eggs, the plant also serves as a primary food source, says Dr. Jayne Yack, a biology professor at Carleton University.

Due to habitat loss, monarchs have remained on Canada’s list of species of “special concern” since 1997.