Close up of snowy owl looking at camera and sitting on the snow, with wings up and back
Snowy Owl: In recent years, snowy owls have shown up in Wisconsin in large numbers. © Jocelyn Praud/TNC Photo Contest 2019

Stories in Wisconsin

December Wisconsin Nature Notes

In December, mammals are MADly adapting to winter in different ways, and snowy owls and finches may venture south from Canada into Wisconsin.


Few birds are as revered and as keenly awaited every winter as snowy owls. More than a century of scientific study has shown that their large flights into Wisconsin, called irruptions, occur periodically every four or five years. But in recent years, an unprecedented number of snowy owls have irrupted in remarkably large numbers, obliterating what was thought of as “normal” and confounding researchers. These irruptions have occurred not only in Wisconsin, but across the eastern two-thirds of the continent.

Historically, a normal winter might produce 30 snowy owls throughout the entire state, while irruption years might exceed a hundred. Those numbers are out the window now. In 2013-2014, 290 unique snowy owls were recorded in the state. The following year, there were 280. And in 2018, roughly 200 snowy owls were spotted across Wisconsin.

Snowy owls irrupt for two major reasons: During years of high lemming populations – their primary prey in the Arctic—a pair of adults may fledge 10 young owls. Once the young fledge, however, territorial adults drive them out of their normal range. In these years, most of the birds seen by Wisconsin birders are juveniles and arrive in good shape.

But we also get invasions in years when the lemming population crashes, thus driving some snowies south. In years of low prey availability, snowy owls won't nest at all or produce very few young. In these years, snowies may arrive hungry and emaciated.

Wisconsin has never experienced this many irruption years in a row. So, what’s going on?

One possibility, linked to climate change, is that a warmer Arctic may be increasing the habitat and abundance of lemmings, allowing snowy owls to produce more offspring more often.

Scientists want to know more and one study, Project SNOWstorm, has placed GPS transmitters on more than 50 snowy owls in the U.S. since 2013 to follow their daily movements. However, the reasons behind the frequent irruptions still may not be known for years.

Snowy owls breed in the treeless Arctic tundra, so when they come down into Wisconsin, they seek out the same open expansive habitat. Look for them in farm fields, near airports, over large wetlands and along coastal areas.

Close up of a beaver swimming through the water; head and back are visible.
Beaver: In winter, beavers live on food they’ve cached near their lodges. © Ross Geredien


To survive a Wisconsin winter, animals have three choices—migrate, adapt, or die—or MAD. For those animals in the far north of Wisconsin, perhaps it is mad to try and adapt to five months of snow, ice, and below-freezing temperatures, but mammals in particular have found ingenious ways to survive. Only a few migrate, including several species of bats. Deer migrate too, but typically only several miles to find optimal winter habitat under the cover of conifers.

Some species migrate downward into dens—chipmunks, raccoons, bears, and skunks go into torpor wherein they lower their body temperature and slow their heart rate and respiration dramatically, decreasing their metabolic rate up to 95 percent.

Woodchucks go into true hibernation under the ground, dropping their body temperature to near freezing and breathing only about once every six minutes.

Beavers and muskrats wait out the winter in lodges and huts that they build. The walls of a beaver lodge and of a muskrat house are nearly a foot thick and help to keep the winter cold at bay.

Beavers put up a store of food to get them through the winter. This usually amounts to several hundred tree branches anchored in a heap in the sediments. The beaver’s cache of food represents a winter process called “larder hoarding,” a practice no different than we humans canning and freezing enough food to get us through the winter. Other species engage in larder hoarding too, like ground squirrels and chipmunks, which cache large quantities of seeds in their dens, or red squirrels, which store pine cones in storage middens.

Snowshoe hares finalize their "change of coats" in early December, becoming pure white except for the black tips of their ears. The hind feet of a snowshoe hare are four to six inches long, and they're attached to a body that's just 15 to 20 inches long. If our feet were proportionate in size to the hare's feet, they would be nearly 30 inches long, making them a size 45!

Bat species that don’t migrate need an environment much like a refrigerator, one cold enough to reduce their metabolism to near zero, but not freezing. For most bats, that means hibernation in caves or old mines, not attics, though the big brown bat is known to overwinter in walls and attics, the only Wisconsin bat to commonly do so.

Rodents like mice, voles, moles, and shrews hang out all winter in the subnivean environment, the zone underneath the snowpack. Once the snow cover is six inches deep or more, the temperature stays just above freezing regardless of the temperature outside.

Every mammal species has a winter story to tell, and all of them are remarkable!

Close up sideview of blue and yellow male Kirtland’s warbler perched in a jack pine tree
Kirtland’s Warbler: These rare warblers migrate more than 1,500 miles from their nest sites in Wisconsin to spend the winter in the Bahamas. © Doug Greenberg/Flickr CC 2.0


In December, Wisconsin’s Kirtland’s warblers are hopefully living the good life in shrubby thickets in the Bahamas, having made the 1,500-mile migration in September from their nest sites.

Always a rare bird, until 1995, Kirtland’s warblers had only been known to nest in the northern part of Michigan’s Lower Peninsula. Today, they also nest in the Upper Peninsula, and since 2007, have nested in Wisconsin and Canada. That’s good news for this little songster that almost went extinct in the 1970s.

In 2018, 51 individuals were confirmed in Wisconsin, a banner year for the Kirtland’s warbler in the state. Between 42 and 47 nestlings successfully fledged from 11 nests. If all of them survive the rigors of migration back to Wisconsin, it bodes well for the recovery of this tiny warbler.

Today, Kirtland’s warblers face two significant threats: lack of crucial young jack pine forest habitat maintained by fire and the parasitic brown-headed cowbird. To nest, a pair of Kirtland’s warblers requires at least eight acres of dense young jack pine forest 5-to-20 feet tall and 6-to-22 years old. Kirtland’s warblers nest only on the ground near the lower branches. As jack pine trees grow older, land managers must continuously create new habitat.

The Nature Conservancy has been part of the effort to bring the Kirtland’s warbler back from the brink of extinction.

The Conservancy is using controlled fire and other management tools in Michigan to protect and restore the fire-dependent jack pine forests that the birds need to breed and nest successfully.

In the Bahamas, the Conservancy is part of a team that has substantially increased scientists’ knowledge of the bird in its winter home by providing field experience and training to Bahamian biologists while studying the winter habitat requirements of the bird. The more we know about the Kirtland’s warbler’s wintering habits, the more we can do to protect the places it needs to thrive.


Every winter, bird watchers in Wisconsin excitedly await the arrival of birds that don't normally winter this far south. Depending on the year, food scarcities in Canada send one or a few species to Wisconsin in often dramatic movements called irruptions.

Even better, once a decade or so climatological patterns and bird numbers combine for a super flight that sends six, seven, or eight northern species deep into the central and southern United States, and that’s when things get exciting.

The birds most commonly associated with these periodic winter irruptions are the winter finches – Pine Grosbeak, Red Crossbill, White-winged Crossbill, Purple Finch, Pine Siskin, Common Redpoll and Evening Grosbeak.

The arrival of winter finches doesn’t necessarily indicate a harsh winter ahead for us. Trees generate seeds in annual cycles, producing massive amounts of seeds in one year and very few the next. In years with lots of seeds, finches will stay north with more than they can eat. In years without seeds, the birds have no choice but to wander south – they live in the vast boreal forests where there are few people and very few feeders to supplement their diet. Typically, the finches will show up at backyard feeders a little later in winter, ignoring feeders at first in favor of whatever natural food they can find.

Key trees affecting finch movements in the boreal forest are spruces, birches and mountain-ashes. For example, Common Redpolls feed primarily on the catkins produced by birch and alder trees. When catkin production is low, Common Redpolls leave these areas and irrupt into areas where food is more plentiful.

Different birds of prey may also irrupt when seed crops are poor and fail to support the necessary rodents that they prey upon. Other causes for bird irruptions include severe weather that may force birds to find more temperate wintering grounds, or overbreeding, where too many young deplete even plentiful food supplies.

Food shortages are only part of the story. Scientists have recently pinpointed a climate pattern that likely sets the stage for irruptions. Persistent shifts in rainfall and temperature drive the boom-and-bust cycles in seed production, which in turn drive the mass migrations of Pine Siskins, the most widespread of the irruptive migrants. This discovery may make it possible to predict irruptions more than a year in advance.