The hearts of lepidopterists in Alberta are fluttering over a new kind of royal visit.

An unprecedented migration of monarch butterflies has brought the fragile, orange-and-black fliers north all the way from their wintering grounds in Mexico in numbers that lovers of the brightly coloured insects have never seen before.

"They've been reporting monarchs non-stop for the last week-and-a-half," said an excited John Acorn, a University of Alberta biologist and chairman of the Alberta Lepidopterist Guild.

"Before last week, I'd only seen one monarch in Edmonton in my entire life. I'm now up to nine."

Monarchs are undisputed kings of winged wandering in the insect world. Despite a wing span typically less than 10 centimetres, they commute from their wintering grounds west of Mexico City thousands of kilometres north to Canada and the U.S.

The journey takes three generations. The monarchs that return south are the grandchildren of the ones that left it.

'Legendary' migration

"The migrations are legendary," said Acorn, also a well-known broadcaster and science educator. "It's really quite marvelous."

Almost all monarchs end up along the eastern half of North America. Those that head west rarely venture further than southern Alberta.

Two monarch butterflies vie for space on a blue phlox blossom. (Sean D. Elliot/The Day/Associated Press)

"It's super-rare in Edmonton," said Acorn. "It's the biggest migration, the furthest north, in let's just say recorded history."

Acorn says that means the butterflies have met with favourable conditions along their journey. Although the number of monarchs leaving Mexico has been declining in recent years, the fact the trip takes more than one generation means they can rebuild their numbers along the way.

Other butterfly species have also been showing up in big numbers across the country. The website ebutterfly.ca, which records all sightings by butterfly lovers, reports this year's migration is "breathtaking and probably unique" in the last 140 years.

"This year is going to be one for the history books," says the website, which is run by scientists and environmental groups.

Mysterious monarchs

The reasons for this year's bounty are mysterious, said Acorn. Because monarchs range over such a wide area, it's hard to know just what ecological factor has favoured their northward movement in such large numbers.

Some have suggested strong southeasterly winds blew the bugs this way. But Acorn points out Edmonton's monarchs seem to be arriving through central Saskatchewan, not southern Alberta.

It's not the first time monarchs have appeared here. A few were sighted during a 2007 migration, and monarchs have been spotted as far north as Lac La Biche, more than 150 kilometres northeast of Edmonton. But nothing like this.

Acorn saw his first two — doubling his lifetime monarch count — while on a walk through Edmonton's river valley last Thursday. At first, he thought they might have been a captive release as part of a wedding ceremony until he got home and started chatting with like-minded colleagues.

"As soon as I sent my [sighting] record out, I realized. Within the course of that morning, there were all sorts of monarchs around town."

The monarch invasion probably has a few days to run, he said.

Edmontonians can then look forward to a repeat when this year's young emerge as adults in August to start a new journey that may one day see their grandchildren return.

It's a mystery and a marvel, said Katelyn Windels. The 12-year-old saw her first Alberta monarch earlier this month, fluttering about the grasses in a field near her family's greenhouse in Lavoy, about 100 kilometres east of Edmonton.

"It was in our pasture," she said. "It seemed to have some trouble flying — it was just fluttering around in the grass. It's just amazing that it can do that. This little thing that's being blown about by the wind coming all that way."