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Early flyers: two monarchs in Mexico, in February 2009, fuel up for the journey north.

Millions of monarch butterflies have begun their epic migration from the Great Lakes down to Mexico for the winter, and Point Pelee, Ont., Canada's southernmost landmass, is once again a focal point for fluttering orange-and-black wings.

Staff at Point Pelee National Park and amateur lepidopterists closely monitor the insects' mystical migration each year to get a handle on trends in population size.

World's greatest migrators

Sooty shearwaters: 64,000 km round-trip from New Zealand to California and Japanese coasts.

Arctic terns: 34,000 km, Arctic to Antarctic, but no one sure if same year.

Grey whales: 16,000-22,000 km, round trip from Mexico's Baja Peninsula to Arctic.

Indian dragonflies: 14,000-18,000 km, round trip from India to southern Africa.

Monarch butterflies:  4,000 or so km, round trip from Mexico to central Canada.

Ruby-throated hummingbirds: 6,400 km, round trip from Central America to north-central Canada.

Caribou: 6,400 km, longest overland migration, across the Arctic tundra.

The daily monarch count starts each night at 7 p.m. and runs until the end of the month, said Meaghan Ruston, the park's promotions officer.

"We just head on down to the tip and count how many monarch butterflies we can see," she said.

The monarch migration to Mexico, a round-trip journey of up to 4,500 kilometres, has piqued scientific curiosity for decades. It was known in ancient times that great waves of monarchs headed south with the arrival of cold weather, but only in 1975 did researchers confirm that the butterflies were wintering in the land of Juarez.

Adding to the mystique is that no one butterfly makes the complete round trip. It takes four or five generations of butterflies to complete, meaning the creatures have some mechanism that guides them to the same southern mating ground year after year.

Antennae hold the secret

Researchers pieced together most of the conundrum in the last year.

Biologists knew monarchs have a compass in their brains that lets them navigate according to the position of the sun. Because the sun moves across the sky through the day, however, scientists contended the butterflies also needed a molecular clock to compensate and keep flying in the right direction.

A study at the University of Massachusetts unravelled the nature of that clock last fall. The time-tracking is based not in the monarchs' brains, as had been hypothesized, but their antennae, without which they fly in random directions.

The thousands of monarchs passing through Point Pelee will be tracked along their journey at 1,100 butterfly way stations in Canada and the United States.

Their migration was once thought to be the longest in the insect world, but evidence from a handful of Indian dragonflies suggests they mount an 18,000-km journey through the Maldives to southern Africa.