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AS AUTUMN 2013 drew to a close, naturalist Scott Weidensaul hung up his outdoor gear and settled down for a quiet winter at home in Pennsylvania. He was writing a book about owls and planned on finishing it before nature perked up again in spring. But he hadn鈥檛 reckoned on the biggest invasion of snowy owls for a century. As reports of sightings flooded in from eastern Canada and the north-eastern US (see diagram), Weidensaul and other owl enthusiasts swung into action. 鈥淲e all knew we鈥檇 never get such an opportunity again,鈥� he says.

Most people will never see a snowy owl in the wild, but its distinctive white plumage and golden eyes are familiar to millions thanks to Hedwig, Harry Potter鈥檚 message-carrying pet. 鈥淭hey are one of the world鈥檚 sexiest birds. Everyone recognises them, but there鈥檚 so much we don鈥檛 know about them,鈥� says Weidensaul. The reason is simple: snowy owls inhabit the high Arctic, where summer is brief and winter long, dark and lethally cold. For eight months of the year fieldwork is impossible in the region, so the owl鈥檚 winter habits have long been a mystery.

Snowy owls spend summer at the very highest latitudes, from North America to Siberia, via Greenland and Scandinavia. A top predator of the tundra, their success is intimately linked to the regular boom-and-bust population cycles of their summertime prey: lemmings. When lemmings are plentiful, owls raise many chicks. When scarce, they may not breed at all.

Such an unpredictable food supply has made nomads of snowy owls. While most other birds return to the same place to nest each year, snowy owls roam the tundra searching for somewhere with an abundance of lemmings, and before settling on a site.

In autumn, as the lemmings begin to disappear beneath deep snow, some owls fly south where the hunting is easier. Sometimes they migrate en masse, a phenomenon known as an 鈥渋rruption鈥�. These are unpredictable and can be triggered by many things, but they are especially common in years when owls breed successfully, increasing pressure on the Arctic鈥檚 limited resources. Every four years or so there鈥檚 an irruption and every few decades there鈥檚 a big one.

鈥淥ne snowy owl in Florida attracted so many sightseers it was given its own wildlife ranger鈥�

In the summer of 2013 the Salluit region of northern Quebec saw a massive explosion of lemming populations, which attracted owls from across the Canadian Arctic. 鈥淓ach pair produced seven or eight fat chicks instead of the usual one or two,鈥� says Jean-Fran莽ois Therrien of the Hawk Mountain Sanctuary in Pennsylvania, who spends his summers studying owls in the Canadian Arctic. By early December, thousands were pouring south across the Canadian border into the north-eastern US and the Great Lakes region. During normal irruptions, birds rarely get further than Cape Cod and New York. This time, some travelled as far south as Georgia, Florida and even Bermuda. 鈥淵ou have to go back to 1926/27 or perhaps even to the 1890s to see the same size irruption,鈥� says Weidensaul.

As the number of sightings snowballed, Weidensaul, David Brinker and Steve Huy, who coordinate a network of owl researchers called Project Owlnet, leapt into action. 鈥淲ith no advance warning, there had been no time to plan research or apply for funding,鈥� says Weidensaul. Yet within a week, Project SNOWstorm was up and running. 鈥淲e were riding a wave of publicity. Everyone knew about the owls. The one in Florida was attracting so many sightseers it was given its own wildlife ranger.鈥� Dozens of wildlife experts, owl researchers, bird banders, vets and pathologists volunteered their services. Bird groups and ornithological organisations donated cash to get the project started and crowdfunding raised enough money to buy tracking devices. Donations flooded in from as far afield as Spain, Norway and the UK.

The aim was to squeeze as much information from the irruption as possible. Thousands of people sent in photos to help identify the sex and ages of the owls. Experts were on call to perform autopsies and tissue analyses on casualties found by the public, checking their condition and measuring levels of toxins and pollutants.

The key to that essential mystery of what owls do in winter, however, lies in . Before winter ended, the team had captured 22 owls in seven states, from Minnesota to Massachusetts, and fitted them with state-of-the-art transmitters that would follow their every move (see 鈥�Hedwig phone home鈥�).

From the start, it was obvious that snowy owls weren鈥檛 going to conform to expectations. 鈥淲hat we thought we knew about this part of the owl鈥檚 life cycle we didn鈥檛,鈥� says Therrien.

The most oft-repeated assumption to bite the dust is that snowy owls are diurnal 鈥� active during the daytime. The tracking data showed clearly that they rest by day and hunt and migrate under cover of darkness. 鈥淵es, they hunt in daylight in the Arctic summer,鈥� says Weidensaul. 鈥淏ut only because it never gets dark.鈥� Given a choice, they are as nocturnal as most other owls.

Nor, as supposed, are irruptions triggered by starvation. 鈥淲e could see these owls were fat 鈥� comically fat,鈥� says Weidensaul. Instead, Therrien suggests, when there are so many owls, the experienced adults probably monopolise territories closest to the breeding grounds, forcing younger birds to move elsewhere. The photos sent in by the public indicated that almost all the owls flying south that winter had been born that summer.

As seasoned snowy-watchers, the team wasn鈥檛 entirely surprised by these findings. What did astonish them was where the owls went in search of food. 鈥淲e thought we鈥檇 see them feeding mostly on open farmland where there are lots of rodents and rabbits,鈥� says Therrien. The owls had other ideas.

Owls in the Great Lakes region seemed fixated on ice, moving onto the frozen lake surface for weeks on end. 鈥淎t first we couldn鈥檛 imagine what they were finding to eat,鈥� says Weidensaul. But by comparing their movements with satellite images of the lake鈥檚 surface, the team realised the owls were focusing on cracks in the ice created by shifting winds 鈥� slivers of open water that harbour large numbers of waterbirds, especially diving ducks.

Although unexpected, this fitted with . He and colleagues at Laval University in Quebec found that some of the owls they tagged during their summer studies in the Arctic had headed north for the winter and spent weeks on the Arctic ice pack, where they could only be feeding on sea ducks.

Owls that spent winter 2013/14 along North America鈥檚 Atlantic coast also had a taste for waterbirds. 鈥淪ome hunted exclusively over water, sometimes using channel markers and buoys as hunting perches,鈥� says Weidensaul. Tracking data can鈥檛 reveal what they were catching, but there were clues in the pellets they regurgitated after feeding. Those from some owls around the Great Lakes contained nothing but the indigestible remains of waterbirds. Those found by the coast included the feathers of ducks, coots and gulls.

Inland, some owls were hunting rodents and rabbits, as expected. But a few were more ambitious. In the Amish country of south-central Pennsylvania, the open farmland attracts great flocks of snow geese in winter. Although bigger than snowy owls, some still became prey. 鈥淲e shouldn鈥檛 be so surprised,鈥� says Therrien. 鈥淟ook at the size of the owls鈥� feet 鈥� they are far too big just to catch lemmings and mice.鈥�

So, snowy owls are unexpectedly flexible about where they breed and what they eat. 鈥淭his is an enormously adaptable bird,鈥� says Weidensaul. That could prove important to a species facing an uncertain future in a rapidly warming Arctic. Already there are signs that changing climate is with . To make matters worse, the global population of owls may have been seriously overestimated. Long thought to number some 300,000 birds, suggest there are more like 30,000. 鈥淭heir future is very much in question,鈥� says Weidensaul.

Project SNOWstorm might help to answer some of the key questions. To everyone鈥檚 surprise, the owl invasion of 2013 was not a one-winter wonder: 2014 saw an 鈥渆cho-irruption鈥�, with a new batch of young joining many of the birds that had flown south the previous year. So, instead of spending last winter analysing data from the mega-irruption, the team turned out to capture and tag another dozen owls and get re-acquainted with old friends.

鈥淭hrough Project SNOWstorm we hope to understand a big chunk of the owl鈥檚 life history that until recently had largely been a blank slate,鈥� says Weidensaul. 鈥淭hat鈥檚 a good first step towards knowing how we can preserve these birds in the years ahead.鈥�

(Images: Audrey Robillard, Louis-Marie Preau/Naturepl.com, Alan Richard)

This article appeared in print under the headline 鈥淲hen owls go south鈥�