Butterflies have migrated across Eastern Canada this spring in unprecedented numbers, reflecting the warm winter throughout North America and raising alarm bells about what it might mean for other species.

"It's probably the most exciting year for butterflies that Canada has ever seen," said Jeremy Kerr, a professor of biology at the University of Ottawa.

Estimates from the field suggest there are 300 million red admiral butterflies from Windsor to New Brunswick — more than 10 times what would be seen in a typical year. Numerous other butterfly species have also arrived or appeared in greater numbers, Kerr said.

What are red admirals?

Red admiral butterflies, Vanessa atalanta, are found throughout North America. Smaller than a monarch, their black, orange (some say red) and white wings are between about 4.5 and 5.7 centimetres wide. Red admirals are extremely adaptable and can sip nectar from almost any kind of flower. They are often seen in gardens. 

The explosion in butterfly populations is mostly due to the warm weather this winter throughout North America. It caused butterflies to emerge from their overwintering forms in places like Texas and Florida sooner, and then improved their survival rates as they headed north.

The first local butterflies (ones that overwinter in Canada) were seen as early as March — "damned peculiar," Kerr said —and southern Ontario was hit by its first big wave of migratory butterflies in mid-April, thanks in part to strong winds. The red admirals moved so far north so early that they flew into areas with snow, something that normally wouldn't happen.

They were also joined by the painted lady, a closely related butterfly.

A second wave arrived in early May, boosting the diversity in southern Ontario, especially from Windsor to Toronto. Kerr said one enthusiast recorded 22 different species in a single Windsor park. The blog of the Royal Ontario Museum in Toronto noted the appearance of such species as the American lady, question mark, mourning cloak, cloudless sulphur, grey hairstreak, variegated fritillary and American snout.

How do you count butterflies?

Researchers take a best guess based on reports from the field, surveying particular sites and extrapolating a larger number from the count. For instance, Kerr recently surveyed a 100-metre wide location for red admirals, counting 102 flying overhead in an hour. Biologists collect such information from scientists and enthusiasts to keep track of the leading edge of migrations and how many butterflies are likely involved. This year, the University of Ottawa started an "eButterfly" hub that allows "citizen scientists" to post their observations and photographs.  

Felix Sperling, a biologist at the University of Alberta, said the early and large migration has been a strictly eastern phenonemon, save for some red admirals approaching the southern border of Manitoba. He said the red admiral is known to have surges — and years where they're no-shows — but in Western Canada they tend to have their own ebbs and flows in numbers.  

Kerr said the butterflies won't have any immediate effect on their environment — the bigger question is what it might mean for other animals.

Butterflies are excellent indicators of environmental changes. "Is it a really beautiful sign of something really worrying to come?"

However, Sperling and Kerr both said it's impossible to know what the change means.

"We don't know yet which species are going to be winners and which are going to be losers in this time of rapid environmental change," Kerr said.

Two immediate questions yet to be answered are how abundant monarch butterflies will be when they arrive (usually in early to mid-June), and how plentiful the red admirals (and their larvae) will be when they give birth to a second generation this summer.