How Will the Polar Vortex Affect Wildlife?

January 8, 2014

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Many wildlife species, like snowshoe hares, are well adapted to survive cold temperatures. Photo: Christopher Brown
Many wildlife species, like snowshoe hares, are well adapted to survive cold temperatures. Photo: Christopher Brown

Update. January 27, 2014. The “polar vortex” may have passed, but frigid temperatures continue throughout the United States. This blog offers a look at how some wildlife might be affected. I’ve also added a section on forest pests since I’ve gotten a lot of inquiries about that.

The “polar vortex” that is bringing frigid temperatures to much of the United States is miserable for people.

But how do wild animals cope with these extreme conditions?

Many wildlife species, of course, are well adapted to thrive in even the coldest of temperatures.

Wild animals have ingenious ways of conserving energy, including hibernation, torpor and other physiological changes. Some simply migrate to warmer climes. Others have thick layers of fat or lush fur that helps them stay warm and dry.

But there are more unusual ways of staying warm. Ruffed grouse, for instance, will tunnel completely into the snow – in essence forming a warm igloo safe from cold temperatures and predators.

Naturalist Bernd Heinrich reports flying squirrels cramming into small tree holes, huddling together in a tight mass for warmth. (Heinrich’s book Winter World is a treasure trove of stories of how animals survive the cold).

There are species, though, that have recently expanded their range due to climate change or other factors. When a cold snap occurs, they’re ill prepared for it.

While it’s still too early to tell how this week’s weather will affect wildlife, research suggests some species will not fare well. Here are a few likely to be particularly vulnerable to the polar vortex.

Carolina wrens expand their range northwards until cold weather causes die-offs. Photo: Ken Thomas
Carolina wrens expand their range northwards until cold weather causes die-offs. Photo: Ken Thomas

Carolina Wrens

While visiting my parents and brother in central Pennsylvania over the holidays, I enjoyed watching the antics of the Carolina wrens visiting their bird feeders. They’re fun birds to observe, always active and vocal.

They’re also somewhat unfamiliar to me: growing up, we’d never see them around the yard. Indeed, Carolina wrens have been expanding their range north over the past decades. They are not migrants: they live in a locale year round.

However, these birds don’t fare well during periods of very cold weather. Since they don’t migrate, they often perish.

This pattern has been recorded by the Great Backyard Bird Count, a citizen science initiative held each February where participants keep track of birds seen in backyards or nearby parks. In years with very cold weather, Carolina wren sightings diminish greatly — the birds have died off, and their range contracts.

The Carolina wren population eventually expands northwards again. It takes time. According to Great Backyard Bird Count reports, it can be ten years or more until Carolina wrens return to northern states in numbers following a cold weather die-off.

My parents tell me they have not seen the Carolina wrens this week, not a promising sign. The polar vortex will likely wipe out many of these birds in the East and Midwest. They’ll be back, but not immediately.

Opossums are prone to frostbite on their hairless tails. Photo: Chris Helzer/TNC
Opossums are prone to frostbite on their hairless tails. Photo: Chris Helzer/TNC

Virginia Opossums

The opossum has been expanding its range north for decades. It’s particularly well adapted to humanity, able to survive amidst farms, city parks and suburbs.

It’s much less well adapted to winter weather.

Unlike some other mid-sized mammals that thrive in the presence of humans – think raccoons, red foxes, coyotes – opossums don’t have furry, protective tails. In fact, their tails (and ears) are hairless, making them particularly susceptible to frostbite and even hypothermia.

Many opossums bear physical evidence of  surviving harsh winters – damaged ears and tails. Their tails often appear stumpy or as if something had bitten them off, but these are signs that their tails suffered frostbite.

Opossums will den for a few days to escape the cold, but they don’t hibernate. They have to feed periodically. Sometimes they’ll change their nocturnal habits and feed in daylight hours during the winter, to take advantage of warmer temperatures.

That doesn’t make much difference, though, when it’s -20 degrees Fahrenheit (as it was in parts of their range this week).

Expect to see a lot of tail-challenged opossums this spring.

Manatees require warm springs to get through cold spells. Photo: John Winfree/TNC
Manatees require warm springs to get through cold spells. Photo: John Winfree/TNC


Perhaps hardest hit are animals like manatees that thrive in more tropical environments, but encounter chilly weather in the northern parts of their range. Florida, home to well-known populations of manatees, is also experiencing a cold spell this week.

Manatees require water 68 degrees or warmer. Despite looking rather plump, they  have very little fat to protect them in cold water.

When they swim in cold water for long periods, they suffer from what is known as manatee cold stress syndrome – leading to starvation and death.

There are current reports of hundreds of manatees moving into warmer springs that remain a constant temperature.   Fortunately, these springs are being closed to public access so the manatees can take refuge there without being further stressed by people.

These winter warm-water areas are vital to the continued survival of manatees in Florida.

Non-native green iguanas fell from the trees during Florida's freezing temperatures in 2010. Photo: Matt Miller/TNC
Non-native green iguanas fell from the trees during Florida’s freezing temperatures in 2010. Photo: Matt Miller/TNC

Non-Native Reptiles

In Florida, species like Burmese pythons and green iguanas can become invasive, posing serious threats to native wildlife. When these species are released into a new environment, they find lots of prey but none of their native predators. Their populations can grow rapidly.

Still, they’re not equipped for cold weather. Most conservationists consider this a good thing.

During the so-called “deep freeze” Florida experienced in 2010, there were frequent sensational media stories of iguanas dropping from trees.  Most of these iguanas weren’t actually dead; they enter a catatonic state when the temperature drops to 37 degrees Fahrenheit.

There were similar stories of pythons dying by the thousands, although this may have been overstated – there are still plenty of non-native snakes in South Florida.

Zoologist Vladimir Dinets reports in his new book Dragon Songs that non-native spectacled caimans (a crocodilian species) pretty much disappeared from the state following the freeze.

It’s interesting to note that native alligators did just fine. But Dinets reports that dozens of American crocodiles, a native species, died due to the cold in 2010.

Cold weather can kill invasive forest pests, but please don't move firewood to new locations -- surviving pests can still be lurking there. Photo: Leigh Greenwood/TNC
Cold weather can kill invasive forest pests, but please don’t move firewood to new locations — surviving pests can still be lurking there. Photo: Leigh Greenwood/TNC

Forest Pests

Frigid temperatures might have another positive effect: killing invasive forest pests. These non-native pests can hammer forests but cold weather offers some hope of slowing the spread.

As Minnesota Public Radio reports, the emerald ash borer could suffer through the kill of a high percentage of emerald ash borer larvae. Since it was first confirmed in southern Michigan in 2002, the emerald ash borer has spread rapidly — now infesting more than 100,000 square miles. It devastates ash trees, and there’s no hope of eradication. But at least the cold weather might slow the spread, buying conservationists more time.

The Don’t Move Firewood blog also reports that the hemlock woolly adelgid, an insect devastating to eastern forests, is affected by cold weather. However, the  temperature needs to drop below -22 degrees Fahrenheit before this pest begins dying in numbers.

Leigh Greenwood of Don’t Move Firewood notes that even the coldest weather won’t kill all forest pests, it will merely reduce their numbers. So please don’t move firewood to new locations; there may still be (living) invasive species lurking there, and you could spread the pests to new locations.

It will be interesting to learn what effects the “polar vortex” has on native and non-native wildlife. Are you seeing any impacts on wild creatures near your home? Your observations can help scientists learn more about how species fare in this cold weather.

Matt Miller

Matt Miller is director of science communications for The Nature Conservancy and editor of the Cool Green Science blog. A lifelong naturalist and outdoor enthusiast, he has covered stories on science and nature around the globe. Matt has worked for the Conservancy for the past 14 years, previously serving as director of communications for the Idaho program. More from Matt

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  1. Will the cold decrease the deer tick and Emerald Ash Borer population?

  2. Unfortunately, introduced caimans didn’t die out completely in 2010. A few are still present in Homestead area of Florida despite decades of attempts to exterminate them.

  3. Re the Carolina Wren … personal observations (in Montreal where they are a relatively recent and still precarious arrival) and a reading of the limited literature are leading me to conclude that the limiting factor for CARW expansion northwards in cold winters and the cause of the occasionally observed die-back is only coincidentally the temperature. More importantly is the fact that they are adapted for finding food around their feet in the leaf litter of forest floors and when there is deep, crusty snow they cannot feed … unless they find a secondary food source such as a well stocked garden feeder. Several pairs in my immediate vicinity are thriving around feeders with nearby brush-piles etc in which they can shelter. Recent temperatures of -25C+ do not seem to be troubling them.

    1. I’m in NW Arkansas. When the artic weather first hit, my local Carolina Wrens were feeding on the suet I have out, but we had 48 hours with temps below 15 degrees and 72 hours or more below freezing. I haven’t seen or heard the wrens since. This article makes me fear for them, although obviously it wasn’t as cold here as it was in more northern climes.

  4. Great as usual. Luckily for us we dodged the polar vortex. In Idaho we had a major die off of upland bird broods from lack of water and to much heat last spring and summer. It was interesting to see arial images of Manatee gathering at the outlet of a Nuclear Power Plant for warmth … wonder if they will glow in the dark now :-}

  5. Carolina wrens in my area of western Ma. have been doing well during the very cold temps because I feed them beef suet and sunflower seeds. They have been in my area for a few years and have had several successful births in one of my backyard birdhouses. Suet is very important to their survival.

    1. Thank you for your comment, I normally see them in my yard where I keep 10 different kinds of feeders, 2 to 4 suet, I’ll be sure that the suet feeders are plentiful. I have been feeding so many birds that I need to replenish some of them in the afternoon. It worries me.

  6. We had extremely cold and snowy winters in Dayton, Ohio 1974-1975 and 1975-1976. Carolina wrens (and bobwhite quail) were completely wiped out. They were not back by the time I left for Minnesota in summer of 1978.

  7. Thank you for this article. I have been wondering about the affect of the low temperatures on wildlife, but this is the first documentation I have seen on this subject.
    We only had 1 Carolina Wren on 30 January, none on 19 January 2014. We are doing a count today. Thank you for the information on suet.
    I have not seen any of our opossum families since the temperatures dropped, and have only seen one raccoon.

  8. “As Minnesota Public Radio reports, the emerald ash borer could wipe out a high percentage of emerald ash borer larvae.”
    Does someone proofread this?

  9. 1329 EST We just saw a Carolina Wren at our feeder area! Happiness!
    Location: east of Dayton. 24F Snow with hard crust!

  10. Definitely have suet out there for the necessary calories a lot of these birds need that fat to stay warm. Even Pizza crusts get polished off… I will intentionally save all pizza crusts for the critters- the chickadees love them!

    Every yard should have some type of evergreen shrub and tree or mountain laurel type bush- just for the necessary windbreak protection wild animals need during storms. Butterflies use them for protection during the Summer storms, and birds will nestle in them during the Winter.

    We humans need to start applying common sense compassion and wisdom on behalf of our “lesser brethren” wild animals.

  11. Anyone know how many Burmese Pythons may be killed by the cold this year?

  12. If the Polar vortex were to come earlier them researchers planned, how would it affect humans and do you think any animals that are tropical, would become extinct or endangered.

  13. I have been seeing a Carolina Wren, mostly at my suet feeder. I was hoping that would carry him thru the very cold spell , but have not seen him in about 4 days. So sad, thinking he didn’t make it. He was s joy to watch.

  14. […] the interest in the recent blog explaining how the polar vortex affected wildlife, this week I’ll look at how three North American grouse species survive the deepest snows and […]