Philadelphians are getting used to large avian visitors to the city.

In addition to Eagles parading down Broad Street, there have been red-tailed hawks nesting on a Franklin Institute windowsill, peregrine falcons hunting from a perch atop a City Hall tower, and the occasional bald eagle swooping in from the suburbs.

But there are guests from farther afield currently living near Philadelphia International Airport.

At least two snowy owls — a young male and a darker, juvenile female — whose nesting grounds are probably somewhere in the Arctic, have been spotted in the area. The first sightings were in December, and it’s thought the PHL stopover may continue through April, when they fly back north. Local birdwatchers have reported as many as four snowies in the vicinity.

Like other large birds or thick flocks, the owls can be a danger to air traffic. But federal wildlife officials have reported that these owls are roaming around the airport perimeter and the riverfront, so they pose little danger of getting sucked into an engine or interfering with a takeoff or landing.

The urban environment, however, can pose many threats.

All about those lemmings

In North America, the snowy owl breeds in open tundra, from Alaska through northern Canada, and the male’s bright white feathers and dark brown highlights blend with its habitat. In winter, they migrate to Southern Canada and the Northern U.S.

But while most birds migrate in predictable patterns, visiting their same vacation spots each year, snowy owls are highly nomadic and unpredictable.

Scott Weidensaul, a wildlife writer and field researcher based in Schuylkill County, explained that the route is determined by the previous summer’s breeding season and the owl’s dependence on its favorite food: lemmings. The lemming population rises and falls in cycles, and when lemmings are in abundance, the size of the snowy population and migration increase in size and dispersion.

“Every four years or so, we get an invasion of snowies coming from the Arctic,” said Weidensaul, who assists in Project SNOWstorm, a collaboration of scientists and veterinarians who track the movement of the birds and work for their conservation.

In 2014, there was a huge “irruption,” aka mass migration, of snowy owls heading south. More than 400 were seen in Pennsylvania that year. By contrast, four years later, just a few dozen have been seen in the commonwealth, mainly around Erie and Lancaster County.

A presence ‘like a person’

The owls tend to follow the I-95 corridor, searching for landing areas with long, open sight lines.

“They tend to end up where they can see around them,” Weidensaul said. “Seacoasts and airports are preferred spots.” The treeless stretches of PHL and the nearby marshes of Tinicum Township and the Heinz Wildlife Refuge, therefore, are excellent hunting grounds for the owls, who fly low to search for prey.

Seeing a snowy owl leaves a lasting impression, per Weidensaul. Besides the owl’s pure white face and sharp yellow eyes, “they are huge birds, with 5-foot wingspans — it’s almost like the presence of a person.”

The owls are also “insanely approachable,” he said. “They’ve never seen any of this before: the planes, trucks, humans… But they become more cautious” the longer they stay in a busy locale.

Dining on pollution

The biggest threats to the visitors at the airport are the planes — being struck in flight or getting too close to a jet blast.

“An idling engine can blast a bird and break every bone,” Weidensaul warned.

Cars also pose a tremendous danger to the owls, who will fly and hunt for rabbits and voles along the I-95 median. After all, “these birds don’t know anything about cars,” Weidensaul noted.

Because they must adjust to the long daylight hours of the Arctic summer, snowy owls are both nocturnal, like most owls, and diurnal, often looking for food during the day, when traffic is high.

Project SNOWstorm has found that migrating snowies are picking up environmental toxins, particularly rat poison, from their food sources. Necropsies of the owls have also revealed toxic levels of mercury, an air pollutant generated by car exhaust and factories. The mercury accumulates in the aquatic food chain; the owls prey on waterfowl that have eaten poisoned fish and other marine life.

An owl in the city can also get tangled in fences or become trapped in an abandoned building where it might choose to roost.

PHL’s snowy owls may stay through the winter, then head back north in mid-March to mid-April, according to Weidensaul. If they can navigate the risks, “a snowy will often settle down if he finds a place he likes.”

But it’s not easy. “There are a lot of dangers they can run into,” said Weidensaul. “It’s a hard life for them.”