Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

Owl in snow. © Gregory K. Scott

Owls feast on other wintering animals such as grouse and rabbits.
© Gregory K. Scott

February 2015

Toughening it out when the temperatures drop

Animals amazingly adapt to Wisconsin's wild winters.

Richard Staffen, Bill Smith and Rori Paloski

It can be tough for humans to stay warm in the winter. Imagine having to go it alone with just the fur, fins or feathers on your back? How do animals survive Wisconsin's winters? We asked some of DNR's conservation biologists, Richard Staffen, Bill Smith and Rori Paloski about animals that have adapted to these cold climates.


Some Wisconsin frog species (such as the leopard frog, green frog and bullfrog), overwinter underwater, buried beneath mud or debris, while others (such as the spring peeper, chorus frog and wood frog), overwinter in leaf litter or under rocks and logs near their breeding ponds and are able to withstand freezing.

One species in Wisconsin, the American toad, overwinters by burrowing into the soil below the frost line. Both the gray tree frog and Cope's gray tree frog are species that can overwinter above ground and are able to withstand freezing conditions. These species have natural, glucose–based antifreeze in their bodies that protects their cells from freezing and bursting. They then can defrost come warmer weather. It is a very amazing adaptation!


For insects that don't migrate (and there are relatively few that do), they can either avoid freezing, or tolerate freezing, by controlling how fast it happens. Avoidance is achieved by picking protected microhabitats like under bark, in the soil or constructed shelters.

Ants are colonial burrowers and construct extensive underground cities which offer protection from the elements. They remain in these burrows until conditions above ground are unfrozen.

There also are many kinds of wasps. Most are predators of other insects. The adults of many species lay eggs in their host where they develop and overwinter underground. Some species, like paper wasps or mud–dabber wasps, build nests above ground. These wasps overwinter as larvae or pupae in cells which were provisioned with food by the adults before they died.


There are a lot of worm species. For earthworms the answer is simple — they live underground below the frost line. Many worms are aquatic and live in environments that don't normally freeze. Many worms are parasitic and live in their hosts, which hopefully have their own winter survival strategy figured out.


Fish survival depends on what kind of frozen waterbody they live in. In rivers there is usually plenty of oxygen because the water is moving and mixing and cold water holds more oxygen than warm water. Lakes can be a different matter. Deep lakes are usually okay unless there is a prolonged period of snow cover and aquatic plants cannot photosynthesize. Shallow lakes frequently experience winter fish kills because there is too little water below the ice. Only very tolerant fish species like mud minnows and bullheads can survive these conditions. All cold–blooded predators like fish have greatly reduced feeding rates during winter.

Backyard birds

Chickadees and other small birds that do not migrate can fluff out their feathers to increase air pockets, adding more insulation. They also shiver like people do, increasing muscle activity and metabolism. They need to continuously feed as they are burning many calories to maintain body temperature. Shelter in conifers and brush protects them from harsh environments. Birds can eat snow but it takes them much more energy to process it. Finding a reliable open water source like a spring or running water is critical. Providing heated water baths is a great way to attract birds to your yard and help them survive.

Geese, turkeys and pheasants

Geese are opportunistic migrants and will move only as far as they need to. Depending on what water remains open during the winter months, they will move south or stay put. Migration has high risk, so if they do not need to migrate they won't. Both turkeys and pheasants can dig for food under the snow, but this becomes increasingly difficult as snow accumulates or ice forms on the ground. Pheasants and turkeys stand a greater chance of survival where they have access to adequate food resources and protective cover.

Bald Eagle Appreciation Days

Bald Eagle Appreciation Days in Prairie du Chien are Feb. 27 and 28. Some hotels also offer Friday educational programs. Other events are co–sponsored by Effigy Mounds National Monument and the Prairie du Chien Chamber of Commerce/Tourism Council. Saturday programs run 9 a.m. to 3 p.m. and are held at Hoffman Hall, Prairie du Chien. Visit Prairie du Chien Area Chamber of Commerce or call (800) 732-1673.


In Wisconsin, snowy owls are usually only seen in late fall and winter and in open areas like fields and along large water bodies. Some of the best places are cities on the shores of the Great Lakes, including Ashland or Superior on Lake Superior and almost any city on Lake Michigan.


The best places to view bald eagles in Wisconsin during winter are near open water where they can find fish. Some of the best places are along the Mississippi River locks and dams and the lower Wisconsin River near Sauk City.

Aquatic invertebrates

Some crayfish live in streams and hide under rocks. The moving water usually has plenty of oxygen. Other crayfish, like the devil crayfish, live in wetlands and overwinter in burrows that they dug down into the water table before things were frozen solid.

Many aquatic invertebrates overwinter in their immature form or adult form and may be gill breathers or grab air from the surface. If the surface isn't available because of ice cover, they may still be fine as their metabolism (and oxygen demand) is extremely low at these temperatures.

Snails don't generally burrow and remain protected, to some extent, by their shells.


Turtles are cold-blooded, which means when they are cold in the winter their body's metabolism slows down. During the winter, turtles will take fewer breaths and also absorb small amounts of oxygen through their skin. Aquatic and semi–aquatic turtles burrow into the substrate in the winter. Wisconsin's one terrestrial turtle, the ornate box turtle, spends all year in sand prairies and overwinters by burrowing itself in the sand below the frost line.


Many mammals will seek out springs and other areas of open water in the winter. If open water is not available, many will eat snow.

Foxes are active all winter and usually hunt by night. They may have burrows dug before the ground was frozen or other comparable daytime shelters.

Like foxes, wolves are active all winter and this is when they hunt in groups called packs. They may sleep together in protected areas, but not in dens.

Mink do not hibernate and can be seen out and feeding throughout the winter.

Deer seek out dense conifers or other shelter to survive winter. These are referred to as deer yards, where they congregate to avoid exposure.

Weather plays a large role in when bears head to their dens, but typically it is in November. They are not true hibernators, are capable of shuffling away if disturbed and can be out during a warm spell. A black bear stores a considerable amount of fat during autumn in preparation for winter torpor, when its heart rate declines from 40 to 10 beats per minute and its body temperature falls from the usual 104 degrees to 91.4 degrees. Females give birth over the winter to young in the den.

Flying squirrels do not hibernate but may utilize torpor during winter and typically take shelter in dens/nests in tree cavities during extreme weather events. They are known to huddle with as many as 20 other flying squirrels to share body heat for survival.

The true hibernators are bats and some mammals in the rodent family like ground squirrels, chipmunks and woodchucks.


Overwintering locations for snakes vary by species, but most overwinter in crayfish burrows, small mammal burrows, old root channels or rock crevices. Snakes feed in the fall before overwintering, but because they are so cold in the winter, their metabolism slows down, and they do not need much food and can go long periods without eating.

Richard Staffen retired from the Department of Natural Resources, Bill Smith and Rori Paloski are conservation biologists in DNR’s Natural Heritage Conservation program.