Photo: Blackpoll Warbler, Owen Deutsch/Audubon Photography Awards
Betsy Martin
Have you ever wondered where animals go in the winter, when many seem to just—disappear?
The ancients puzzled over the disappearance and reappearance of birds. Aristotle thought they transform into other species in winter (which explained why robins appeared just as redstarts disappeared in Greece). Olaus Magnus theorized in the 16th century that swallows bury themselves in clay at the bottom of rivers in wintertime. In the 17th century, Charles Morton offered the most fantastic theory of all; birds flew to the moon and back every year.
Of course, now we know that migration on earth accounts for the disappearance and reappearance of birds and many other species. But you may be surprised to learn that there’s still a lot we don’t know about where various species go.
Blackpoll Warblers (Setophaga striata) spend the summer breeding in the boreal spruce and tamarack forests of Alaska and northern Canada, then in fall migrate up to 5000 miles south to winter in the mangrove forests and rain forests of Brazil.
Blackpoll Warbler, Owen Deutsch/Audubon Photography Awards
In the fall migration they travel south and southeast through Canada and the northeast U.S. and collect along the coastal plain between Nova Scotia and Cape Hatteras, NC. Then, it’s believed—the experts aren’t really sure—the birds head south and southeast over the Atlantic Ocean, and fly nearly 2000 miles to Puerto Rico, the Lesser Antilles or northern South America. This potentially involves an 88-hour nonstop flight over open ocean! To accomplish this, Blackpoll Warblers nearly double their weight before setting out, and they rely on northeast trade winds to direct their flight toward South America.
(Watch Cornell Lab’s animation of Blackpoll Warbler abundance on eBird here.)
(Information from Cornell Lab of Ornithology, Birds of the World)
Butterflies and other insects have multiple over-wintering strategies, some more mysterious than others.
Many insects shelter and are nourished through the winter in micro-habitats under the soil, inside the wood of logs, in trees and even in plant galls. Inactive insects undergo a dormant state, called diapause, in which their growth, development and activities are suspended in winter, with a metabolic rate barely high enough to keep them alive.
Some insects successfully pass the winter as immature larvae, such as the Woolly Bear caterpillar, protected by heavy layers of leaf litter. Other larvae replace the water in their bodies with glycerol, a type of antifreeze. Some grubs simply burrow deeper into the soil to escape the cold.
Dragonflies, mayflies and stoneflies survive as nymphs living in ponds and streams, feeding and growing all winter to emerge as adults in early spring. A few insects lay eggs which survive the winter, such as Praying Mantids.
Some insects, such as the Mourning Cloak butterfly, overwinter as adults in tree holes, leaf litter and under logs and rocks. That’s why Mourning Cloak is usually the first butterfly to appear in Spring. Like some insect larvae, it reduces the water content of its body and builds up glycerol which acts as an antifreeze.
Many butterflies that spend the summer here cannot survive the winter, so they die. Each year, as the weather warms up, butterflies from Mexico and the southern United States fly north to repopulate Virginia such as: Common Buckeye, Cloudless Sulphur, Painted Lady, Red Admiral and others.
Monarch, Judy Gallagher
Cloudless Sulphur, Judy Gallagher
For some species the reverse migration, south in the fall, is more obvious. Cloudless Sulphurs, Mourning Cloaks and Monarchs can sometimes be seen moving southward in groups of thousands. Exactly where all these butterflies go is not always known. Monarchs are the best-known migratory butterflies, yet knowledge is limited even for them. Most Monarchs from west of the Rocky Mountains spend the winter along the California coast while those from central North America spend the winter in roosts in the mountains of central Mexico. But what about the Monarchs from the Atlantic seaboard? Although it seems that many of them also migrate to the same Mexican overwintering sites, others may travel to and through Florida, perhaps flying on to undiscovered sites in the Caribbean or the Yucatan Peninsula. Or, maybe northern Monarchs that enter the peninsula don't survive the winter and for them, Florida is a dead end. Some Monarchs that overwinter in Florida may be largely members of resident, non-migratory populations. At this point, we just don't know.
(Information from North American Butterfly Association, Smithsonian)
Frogs and toads commonly hibernate in winter. After an animal finds or makes a living space (hibernaculum) that protects it from winter weather and predators, its metabolism slows dramatically. It "sleeps away" the winter by utilizing its body's energy stores, then "wakes up" when spring arrives.
Aquatic frogs such as the Leopard Frog (Rana pipiens) and American Bullfrog (Rana catesbeiana) typically hibernate underwater. But they don’t dig into the mud the way aquatic turtles do—they would suffocate. Hibernating aquatic frogs must be near oxygen-rich water and spend a good part of the winter just lying on top of the mud or only partially buried. They may even slowly swim around from time to time.
Terrestrial frogs normally hibernate on land. American Toads (Bufo americanus) and other good diggers burrow deep into the soil, safely below the frost line. Some frogs, such as the Wood Frog (Rana sylvatica) and Spring Peeper (Hyla crucifer) are not adept at digging and instead seek out deep cracks and crevices in logs or rocks, or just dig down as far as they can in the leaf litter. These hibernacula (shelters) are not as well protected from frigid weather and may freeze, along with their inhabitants.
Wood Frog, VA Herpetological Society
And yet the frogs do not die. A high concentration of glucose in the frog's vital organs prevents freezing, even though ice crystals form in the body cavity, bladder and under the skin. A partially frozen frog stops breathing, its heart stops beating and it appears dead. But when the hibernaculum warms up above freezing, the frog's frozen portions thaw, its heart and lungs resume activity and it comes back to life.
(Information from Scientific American)
Various species have amazing and diverse strategies for surviving winter. You can help them.
Blackpoll Warblers don’t breed here, but they do fly over and stop during spring and fall migration. You can help sustain them in their long migration south by planting native, fall-fruiting native shrubs in your yard, such as Spicebush or Viburnum, with fatty berries that help them build up their fat reserves prior to their long oceanic voyage. Avoid using insecticides that kill insects that Blackpoll Warblers and other bird species depend on for food.
To help insects, leave the leaves—leaf litter is key to many species’ winter survival—and don’t be too tidy about cutting and cleaning up flower stems and stalks. Many insects burrow into them for winter protection.
To help the frogs and toads, be careful about digging in winter—you may be digging up somebody’s winter home.
Interested in learning more about protecting and creating habitat for wildlife in all four seasons? Check out Audubon at Home on the ASNV website. Request a visit by an Audubon at Home Ambassador who can advise on habitat improvements you can make on your property.