MARCH MADNESS: MAMMAL REPRODUCTION
We tend to think of March and April as transitional months of very little activity. That may be true for humans, but there's plenty going on in the animal world. Most mammals are either mating, gestating or giving birth during this period. Animals can't afford to wait to mate until the weather warms and the flowers are blooming, because their young would not be born until June or July, often too late for all the growth that must happen before the young of the year face their first winter.
All animals have evolved specific timing for their reproductive cycles. In the squirrel family, gray squirrels have their first litter in March, while red squirrels mate in March and give birth in April. Northern flying squirrels mate in April and give birth in May. In the dog family, red foxes give birth to their kits in late March and early April, while coyotes produce young in April. Timber wolves mate in March and give birth in May.
In the weasel family, nearly all members use delayed implantation to govern their breeding cycle. Mink breed in early March and employ a short, delayed implantation, giving birth in late April and early May. Striped skunks likewise breed in March, but give birth a little later in May. Martens, ermines and long-tailed weasels use an intermediate delay, breeding in July but giving birth in late April to early May. The longest cycle of delayed implantation belongs to fishers and river otters, a cycle that keeps the females perpetually pregnant. Both otters and fishers breed in April, but not until 350 days or so later do the females give birth. Then they promptly breed once again.
March courtship displays are hard to catch, but they're magical when found. Ravens show off their flight skills in courtship loops and dives. Snowshoe hares mate, the males "thumping their hind feet on the [snow] crust, bouncing in circles, and even flipping and twisting high into the air," as naturalist Denny Olson has written. Male grouse begin to drum out messages to females. Chipmunks emerge seeking a partner to court, the male singing at the den entrance to his chosen female in the hope she will like the song.
If the weather warms, male red-winged blackbirds and robins will appear one morning, as if a wand had been waved across the land, their songs jolting human spirits into an expression of thanks for a winter that is at last losing ground.
FIRST DAY OF SPRING AND HOPKINS LAW
The first day of spring (also called the spring equinox) arrives every year between March 20th and March 22nd, which theoretically means we should experience equal periods of day and night. In northern Wisconsin, though, conditions are not quite so equal. The sun will actually rise near 6:01 a.m. and set near 6:10 p.m., granting us nine extra minutes of daylight. However, given the usual continuation of winter, and “cabin fever” well into April, no one would dream of complaining about too much light.
But spring IS on its way. How soon will spring occur in the Northwoods compared to southern Wisconsin? We can accurately predict the date of the first bloom of our northern spring flowers based on Hopkins Law, which says that phenological events vary at the rate of one day for each 15 minutes of latitude and one day for each 100 feet of altitude. (Phenology is the study of the seasonal march of observable biological events – when the first robin returns, when the first trillium blooms, for example.) If we compare biological events in Madison to those in Minocqua, there should be a 20-day interval between the two areas, given their distance apart (about 220 miles) and their differences in elevation.
Thus, if a friend in Madison calls to say she saw her first robin on March 1st, folks in the Northwoods can expect to see their first robin on the equinox —an event which, on average, occurs nearly every year.
Note that variations do occur along the Great Lakes where the weather is moderated by the still icy waters. And Hopkins Law is considered valid only up to June 1st when other factors take control, like available sunlight, soil conditions and rainfall.
Most importantly, everything is now starting to come to life. Buds are breaking, leaves are popping, flowers unfolding – the Great Green Up has commenced. Birds, too, are returning, many following the first hatches of insects. Birds that have wintered in Central and South America, however, will wait until late April and early May to return, fully aware of how fickle winter can still be.
So, while winter lingers, maddening as it often is, it’s time to begin watching for the signs of spring. Many migrating birds utilize the Lake Michigan shoreline as their route north. Consider visiting The Nature Conservancy’s Mink River Preserve in Door County where 1,950 acres are protected and 200 species of bird have been counted in migration and nesting.
To catch the first wildflowers of the year in southern Wisconsin, try Chiwaukee Prairie on Lake Michigan, the last unbroken stretch of prairie of its kind in the state. Chiwaukee is home to more than 400 plant species, including shooting stars, prairie violets and wood betony, which bloom in the spring.
Or consider visiting The Nature Conservancy’s Spring Green Preserve. Known as the "Wisconsin Desert," Spring Green Preserve is a great place to see prickly pear cactus blooming in June.
SAW-WHET OWLS: SMALL BUT MIGHTY…LOUD!
Mid- to late March is the time to go out after dark and listen for the monotonous tooting of a saw-whet owl. While saw-whets produce a series of different calls, the one most often heard is the “advertising call,” which is an endless loop of whistled “toots” on a constant pitch. Even though saw-whets are one of the smallest owls in North America – standing just 5 to 6 inches high with a wingspan of 18 to 22 inches and weighing about as much as a quarter to a half cup of water – their robust call can be heard up to 300 yards away through a forest and over a half-mile away over water.
The call comes at a rate of about two notes per second and sounds like the beeping of a commercial truck that’s backing up or, for science fiction lovers, like your average Martian landing craft. And when researchers say it’s monotonous, they really mean monotonous – the song can literally go on for an hour or more.
Saw-whets also give a call which researchers describe as resembling “the sounds produced by filing [whetting] a large mill saw,” hence the name “saw-whet.” The call is apparently given when an owl is agitated by an intruder.
Saw-whets have a distinctive white "Y" shape on their face, but their bright yellow eyes are their most striking characteristic. As small as they are, they utilize an arsenal of eight sharp talons to take down prey, from small rodents to mammals as large as a squirrel. Saw-whets are said to “have the attitude of a female golden eagle packed into a tiny body.”
Most northern saw-whets migrate southward in winter, concentrating their migration routes along the Great Lakes, but range maps show that they can winter throughout Wisconsin.
They nest in tree cavities, relying on previously excavated holes commonly made by northern flickers or pileated woodpeckers.
The males give their advertising call, the “toot” call, beginning in late January in southern Wisconsin and ending in late May in northern Wisconsin. Their calling peaks at two hours after sunset, so listen around 10 p.m. The call then tends to decrease until just before sunrise.
MOURNING CLOAK SIGNALS COMING OF SPRING
As March wanders in, snowing one day and beaming warmth the next, harbingers of spring appear, from bursting pussy willow buds to the return of the first robin. But few have as remarkable a story as the first flitting butterfly of the season, the mourning cloak. Even with snow on the ground, mourning cloaks will emerge from hibernation, bringing beauty to an otherwise wintery day.
Their emergence seems miraculous on such cold days. They’ve just spent six months nearly frozen in tree cavities, beneath loose tree bark, in wood piles or in unheated buildings. The cold itself is not a direct hazard to the butterflies – rather, it is the formation of ice crystals in their body tissue that can quickly become lethal.
To keep from freezing, mourning cloaks reduce the amount of water in their blood by as much as 30 percent and then thicken it with a sugar solution of sorbitol. Their antifreeze outdoes anything we humans put in our cars. Using electrical conductivity, biologists in Alaska found that mourning cloaks do not freeze until the temperature reaches minus 220°F.
To keep from being detected through the long winter, mourning cloaks fold their wings together so only the undersides, which are drab and serve as camouflage, are exposed to potential predators.
Once they emerge, they are short on fat and need to eat, so they often seek out running tree sap or rotten fruit. As the days become longer and warmer, they’ll mate and lay eggs for the next generation, living only a few weeks. Still, mourning cloaks win the award for greatest longevity among butterflies, living 10 or 11 months.
Butterflies need body temperatures close to ours to fly. All butterflies that are active in spring have dark-colored bodies and wings to aid in solar heating themselves. Watch for mourning cloaks basking, opening their wings and angling their bodies toward the sun to increase their body temperature prior to flight.
The mourning cloak is found throughout most of North America and Europe and in a broad band across central Asia. So, they don’t just announce spring in Wisconsin but around much of the world.