As climate warms, northern flying squirrels are moving out of the Northland

Jun. 19—DULUTH — They are your neighbors that you never see, creatures of the night that spend their days hiding in trees across the Northland and come out only when it's dark.

Flying squirrels are probably a lot more common than you think, found across most of Minnesota and Wisconsin's forested counties and living their lives pretty much unbothered by humans, who are mostly asleep when the big-eyed squirrels are out foraging for food.

But recent research shows that northern flying squirrels, the species that once dominated Northland forests, are being pushed north at an alarming rate — another victim of a warming climate following other species like moose, lynx and spruce grouse.

One study in Ontario found that southern flying squirrels are moving north — and replacing northern flying squirrels — at a rate of about 12.5 miles each year.

The southern flying squirrels "probably get knocked back a little during the occasional hard winters, but that was the average movement," said Michael Joyce, a wildlife ecologist with the Natural Resources Research Institute at the University of Minnesota Duluth. "That's really fast. Especially for such a small animal that doesn't move around much, that has a pretty small home range, that's a really rapid rate of range movement."

Up and out of Wisconsin, Minnesota?

In Wisconsin, northern flying squirrels held the northern third of the state as recently as 30 years ago when wildlife biology students at the University of Wisconsin-Stevens Point commonly recorded them not far from the central Wisconsin campus.

Now, southern flying squirrels have advanced north all the way to Lake Superior, said Rich Staffen, a zoologist in the Wisconsin Department of Natural Resources' nongame program. Northerns seem to be vanishing from most of the state.

"In our trapping in the northern third (of Wisconsin) over the past five or so years, we are commonly catching southerns and only very rarely do we catch a northern," said Staffen, who is somewhat hopeful that the southern's preference for hardwoods may offer the northern flying squirrel some bastion of safety in the region's conifer forests.

Whether the shift is happening as rapidly in Minnesota isn't yet clear. But Joyce is trying to find out, heading a pilot project now to capture both the ultrasonic voices of flying squirrels and to capture their images on trail cameras as well as photo reports from Northland residents.

Northern flying squirrels are listed as a species of concern in Wisconsin and Michigan because of their rapidly shrinking range. So far Minnesota hasn't given the northern flying squirrel any special status. For the most part, the small squirrels have been overlooked by researchers. Now, Joyce hopes to take the first ever comprehensive look at the status of flying squirrels in Minnesota and to confirm if the northward movement of southern flying squirrels is as rapid here as it appears.

Joyce hopes to eventually take a deeper, longer dive into flying squirrel populations, range and trends in Minnesota.

"But first we need some baseline information on just which one is where," he said. "We need to find out what their distribution is across the region now to see how that changes in the future."

Historically, wildlife biologists declared that northern flying squirrels lived in the northern spruce, pine and fir forests of the northeast, while the southern species dominated the hardwood forests of the central and southeastern counties in the state.

"The historic dividing line was about Leech Lake to Pine City," Joyce said.

But southern flying squirrels have blown past that line and now are found in Duluth and likely up the North Shore, where ample hardwoods like oak and maple trees could give them a foothold. Much of the change, more than 100 miles, has happened in the past 40 years.

"We really don't know how far north they have moved. Our goal is to find that out," Joyce said.

Southerns more aggressive, carry deadly parasite

It's not that northern flying squirrels have been hugely impacted by the already warmer temperatures in the region. But the milder winters, especially, have allowed the southern species to move north and, for multiple reasons, when the two species overlap the southern one usually wins, even though they are slightly smaller.

Staffen said southern flying squirrels tend to be more aggressive and will kick northerns out of the best nesting cavities in trees. But perhaps more foreboding for the northern subspecies is that southern species often carries a nematode parasite that, while nonlethal to them, is fatal to northern flying squirrels.

Once again, a warming climate opened the door for a new species to move in, bringing problems with it. It's a similar situation to the northward-moving abundance of deer in Minnesota which have brought a parasitic brainworm with them that's harmless to deer but fatal to moose.

Northern flying squirrels may be losing out to hybridization, too, much like Minnesota's lynx forest cats seem to be losing out as bobcat range moves north. And the trend to more hardwood trees in northern forests also favors the southern subspecies, Staffen noted.

You may remember Joyce as the lead researcher on a project building nest boxes for northwoods fishers. For flying squirrels, his goal is to study 10 different areas across the Northeast comparing trail cameras to audio recorders to see which ones best capture flying squirrel numbers. It turns out that the ultrasonic vocalizations flying squirrels make are picked up well by bat voice recorders, or acoustic detectors. (Joyce said the best bait to lure flying squirrels into trail camera range appears to be peanut butter.)

Joyce said the little squirrels piqued his curiosity when they would show up in trail camera photos taken at fisher nesting boxes he's been placing across the region.

"But what really got me interested in this project was doing some small mammal surveys last fall in an area where I would have expected to see both northern and southern flying squirrels based on their historical ranges in Minnesota. We only saw southerns, which seemed odd to me," Joyce said, noting it became clear that "the ranges were shifting here — as they are elsewhere — but that there didn't really seem to be as much attention to the situation in Minnesota as in Wisconsin and Michigan."

Flying squirrels facts

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There are more than 40 species of flying squirrels worldwide with two of them in Minnesota and Wisconsin — the southern flying squirrel and the northern flying squirrel.

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Flying squirrels do not fly, but glide from one perch to another. Their "flight" is made possible by a fold of skin, a membrane which extends from the front to the hind feet. When the legs are outstretched, the skin stretches out tautly to form a large planing surface which enables the squirrel to glide as far as 150 feet, though most glides are between 20 and 30 feet.

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Living in tree hollows or leaf nests, flying squirrels are the only nocturnal squirrels in Minnesota, seldom seen during daylight. Southern flying squirrels are found mainly in Minnesota hardwood forests, while the northern subspecies occurs in aspen/pine-dominated forests.

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The southern flying squirrel is about the size of a chipmunk, about 9 inches long including the tail and weighing only a couple of ounces. The northern flying squirrel is slightly larger, about 11 inches long and weighing just 3 ounces.

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Flying squirrels are noted for their dense fur, glossy olive-brown above and white below, large brown eyes, and mild disposition. Only the shrews and moles have fur that comes close in softness and silkiness to that of flying squirrels. The upper parts are gray-brown, whereas all the lower parts including the tail are white.

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Both Minnesota species mate in early spring, and about five weeks later females give birth to three to five tiny, blind young. Southern flying squirrels may have two litters in summer, but this rarely occurs in northern flying squirrels.

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Flying squirrels eat a variety of fruits and nuts, insects, small birds and meat scraps. Flying squirrels can be frequent visitors at bird feeders, and some people have lights at the feeders so they can watch the flying squirrel's antics at night.

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Small hawks and owls, foxes, weasels and marten are predators of flying squirrels..

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Flying squirrels, though unprotected in Minnesota, have no meat or fur value and thus are not hunted or trapped.

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Many people who think they see birds flying across highways at night, or around campfires, actually are seeing flying squirrels.

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Flying squirrels do not hibernate but slow their body activity in winter and sometimes nest in groups to stay warm.

Source: Minnesota Department of Natural Resources