White-footed Mice

White-footed mouse from Squirrels and Other Fur-Bearers

Illustration from Squirrels and Other Fur-Bearers by John Burroughs (1909)

“I think mice are rather nice.” So began the children’s poem by Rose Amy Fyleman that I read to my three sons when they were young.

Fyleman was an English writer who lived in earlier times (1877-1957) and her mice were not the primary hosts for the larvae and nymphs of black-legged (Lyme disease) ticks or the possible carriers of hantavirus. Unfortunately, our winsome, large-eyed, big-eared white-footed mice are. They can be covered with the larvae and nymphs of black-legged ticks and build up huge concentrations of the Lyme disease bacteria Borrelia burgdorferi, which doesn’t sicken them, and thus remain the principal reservoir for the disease.

They also enjoy life in our old farmhouse, and we are compelled to keep traps laced with peanut butter especially in early autumn when they are seeking warm winter homes. Live-trapping and moving them are not an option even if we wanted to. The late, great mammalogist William J. Hamilton, Jr. once live-trapped and marked white-footed mice and released them a mile away from their home territory. A few days later they were back, exhibiting an amazing homing ability.

In addition, they know their home ranges of approximately one-fifth of an acre so well that they can quickly find hiding places whenever they are needed.

White-footed mice in an old bluebird box by Mark Lethaby

White-footed mice in an old bluebird box by Mark Lethaby (license)

But as Hamilton once wrote about the white-footed mouse, “It often takes up residence in houses; the first evidence of its presence may be a boot half filled with cherry pits or hickory nuts.” In our house, I find sunflower seed stashes not only in old boots but stuffed under the cushions of our sofas and chairs. And occasionally I’ve uncovered shredded sweaters or shirts balled up into nests on some forgotten closet shelf or in an old bureau drawer.

However, most white-footed mice find homes outdoors in abandoned squirrel, woodchuck, or bird nests, bird nest boxes, tree cavities, half-rotten stumps, rock piles or even in a ball of leaves underground. Recently, researchers have noted that they especially like the humid conditions under the rapidly expanding invasive shrub Japanese barberry, which is also favored by black-legged ticks, still another connection with Lyme disease and white-footed mice.

White-footed mice use their long tails as props and balancing organs, to climb trees, although they are not as adept as their longer tailed congener’s deer mice. White-footed mice (Peromyscus leucopus) and woodland deer mice (P. maniculatus) look alike, and often live in the same habitat in our Appalachian forests. The length of their tails can sometimes be their distinguishing characteristic because the tails of white-footed mice are slightly less than half the total length of their bodies while the eastern woodland deer mice have tails more than half their total length. But the best way to tell them apart is by comparing their skulls. Since I’m not inclined to measure their tails or study their skulls, perhaps some of the mice in our home are deer mice because they too can create havoc inside homes and camps and are carriers of hantavirus, although so far they’ve not been indicted as primary hosts for black-legged ticks.

Both species of mice are abundant over a wide range of North America, but white-footed mice have drastically increased their distribution over the last few decades and now are found in the eastern two-thirds of the United States except for Florida, as far west as portions of Wyoming, Colorado, and eastern Arizona, south through eastern Central America and north in Ontario, Quebec, southern Nova Scotia, and even Labrador.

White-footed mouse by Patrick Coin

White-footed mouse by Patrick Coin (license)

Back in the 1980s, J.O. Wolff did extensive studies of both white-footed mice and deer mice in the Appalachians of southern Virginia and found that female white-footed mice lived in separate home ranges while males had ranges overlapping one or more females with which they mated. It seems as if white-footed mice are usually polygamous, but can also be monogamous, and even in at least one known case polyandrous. These choices may protect their mothers and their young because males can recognize their mates and children and will not kill them. They even often help raise their offspring, although females do most of the work.

Mating begins in early March and continues through late October in Pennsylvania. Their gestation period is usually 23 days unless a female is still nursing a previous litter. Then gestation can be anywhere from five to 14 more days. Most litters contain four or five young but can be from two to eight.

Born tiny and naked, they develop rapidly, and at three weeks of age they are able to leave their nest, although if they are threatened they can leave at 16 days. Previously, if their mother is disturbed while nursing, she runs from the nest with her young clinging to her teats and sometimes falling off as she heads for the nearest hiding place.

These young can breed as early as 46 days of age, but most are two months or 60 days old before they do. Still, it is easy to see how many would survive if they did not have many predators such as skunks, mink, weasels, raccoons, foxes, coyotes, bobcats, black bears, and black rat snakes.

These “most abundant and ubiquitous rodents in Pennsylvania,” according to mammalogist Joseph F. Merritt, also have catholic food tastes, eating whatever fruits, seeds, and small creatures are available. A short list includes various grass seeds, raspberry seeds, shadberries, the fruits of viburnum species, hickory nuts, basswood seeds, and conifer seeds, and in the summer they add meat to their diets in the form of caterpillars, ground beetles, snails, centipedes, occasional small birds, and even other small mammals including young white-footed mice. But their favorite foods appear to be pitted wild black cherry seeds, acorns, and the seeds of jewelweed. The latter taste like walnuts and have turquoise-blue endosperms that turn their stomach contents turquoise-colored in late summer.

northern red oak (Quercus rubra) acorns by free photos & art

northern red oak (Quercus rubra) acorns by free photos & art (license)

On a study of the impact of small mammals on northern red oak regeneration by Colleen A. DeLong and Richard H.Yahner from October 1989 until December 1990 in Huntingdon County, Pennsylvania, they attributed most red oak acorn loss to white-footed mice from 67% to 88% in autumn and 94% to 100% in spring in a section of mature forest where they had planted 400 red oak acorns. Certainly, we’ve had fewer mice in our traps lately, and it may be because we haven’t had a good acorn crop in three years. In fact, many wild fruit and nut crops were sparse or nonexistent in 2013, including wild black cherries.

But by mid-autumn white-footed mice have collected caches of food to get them through the winter. Hamilton discovered that they are also fond of storing clover seed and beech seeds, and once found almost a peck of beechnuts they had stashed in a beech tree cavity in New York State.

Here in Pennsylvania, some white-footed mice have periods of torpor from late December until early February, but most remain active. They stay warm by nesting with other white-footed mice and/or deer mice huddled in tree cavities or by nesting underground.

During all seasons of the year, though, in our home, “they run about the house at night; they nibble things they shouldn’t touch, and no one seems to like them much,” as the poet Fyleman wrote. She may have thought mice were nice, but despite their beguiling appearance, I think they are an attractive nuisance.

The Value of Aging Trees

A big red oak on top of Sapsucker Ridge

A big red oak on top of Sapsucker Ridge

On a hot July day, I sit beneath a large red oak, nestled into a deep buttress, one of several that flare out from this 200-year-old tree. The ground beneath the tree is littered with old acorn remnants as are the bases of the other elders in this stand of deciduous trees.

Protected as a picnic spot by the previous owners, the trees were allowed to prosper even as other portions of our property were cut over in the nineteenth and even twentieth centuries. Instead of fencing a young forest, such as that last 120-acre cutover piece we acquired back in the 1990s, we put up our three-acre deer exclosure in March 2001 around these mature trees.

Now, as I gaze around, I see dozens of oak seedlings and saplings filling in the forest floor. Most are red oaks, but some are offspring of the enormous white oak beside the exclosure gate.

Technically, this is not an old-growth forest but one that is becoming old-growth. Still, it and much of the rest of our property consists of an aging forest. Not long ago, old trees were seen as useless and were harvested so that young trees would grow in their place. But in the last few decades scientists have been discovering that old trees in an old forest are incredible absorbers of carbon dioxide. And red oaks, with their dense wood, are particularly good at absorbing impressive amounts of carbon.

One study, in Massachusetts’s Harvard Forest, found that at 50 years of age, their trees, both oak and maple, were absorbing 0.8 tons of carbon per acre every year. After 15 years, the rate of carbon uptake had doubled. Researchers studying old-growth forests in the West discovered that those forests too absorbed more carbon as they aged.

One of those researchers, Bev Law, of Oregon State University, told a journalist that, “Across forest types, globally, we find that the amount of carbon stored is high in older forests, and that live carbon continues to accumulate for centuries.” This carbon is held not only in the trees themselves but in their fallen leaves and branches as well as in the fertile soil of old forests.

Law is director of the AmeriFlux Network, an international collaborative project founded in 1996 to measure the carbon dioxide, water vapor, and energy in all kinds of major ecological community types or biomes in North, Central, and South America. Using a variety of sophisticated tools, scientists are able to track carbon in any ecosystem. Here in our eastern forests, studies have been done in several states including the aforementioned Massachusetts.

Old trees in an old forest have other uses too. They provide large hollow trees for a variety of birds and animals that use them for nesting, food, and shelter. For instance, sycamore trees are the most massive trees in the eastern United States. They can grow over 100 feet tall and 10 feet wide and live up to 500 years. When they are very old, they have a cavity at ground level large enough, in one recent case, to house a maternity colony of 100 Indiana bats, researchers discovered back in 1993.

A giant eastern hemlock tree in Central Pennsylvania

A giant eastern hemlock tree in Central Pennsylvania

Large, hollow trees also appeal to children of a certain age. I was reminded of Jean Craighead George’s book My Side of the Mountain which my husband Bruce read aloud to all of us when our three sons were young. They were all entranced by the 12-year-old hero of the story, Sam Gribley’s large, hollow tree home in the wilds of the Catskill Mountains. In his case it was a hemlock tree. Remembering the many old-growth hemlocks we have (or had, before the hemlock woolly adelgids killed so many of them in our Pennsylvania old-growth forests), I knew that such trees might have been large enough to house one young boy.

Because our old-growth hemlock trees are mostly in Pennsylvania’s state natural areas, the trees attacked by adelgids have been allowed to die and be reclaimed by the earth as our son, Dave discovered during a recent visit to Snyder-Middleswarth Natural Area. Many people find this wasteful, believing that such trees should be harvested. But Dr. Joan Maloof, who has been writing and speaking about the value of old-growth forests, maintains that “old-growth forests are one of the few land uses where topsoil is created instead of destroyed.”

We like to think that our own younger but mature forest is also creating soil as we allow our trees to fall over and rot, creating, as our young nephew Patrick once cried over and over, “Dirt, dirt, dirt” while sifting the powdery, reddish-brown remains of red oak branches through his fingers. Those powdery remains of the heartwood and sapwood appear to be a nursery for the downy rattlesnake plantains I’ve discovered inside and outside our exclosure. When we first put up the exclosure, I found a large colony of this orchid growing on a slope where several trees had rotted down to heartwood and sapwood. Another plant that we found outside the exclosure and fenced had germinated beneath a stump that was seeping rotted wood at its base.

Then, a couple years ago, I found still another downy rattlesnake plantain plant that had germinated on the decaying remains of a fallen red oak limb. At the same time, I noticed that the fenced plant outside the exclosure was looking poorly. I gathered up more of the “natural” fertilizer from the trunk and sprinkled it over the plant, and once again it is thriving. I haven’t seen any studies that indicate that downy rattlesnake plantain needs this material, but I wonder.

Photographing downy rattlesnake plantain in our deer exclosure

Photographing downy rattlesnake plantain in our deer exclosure

As our forest has aged over the last 41 years we’ve lived here and now nearing one hundred years of age in our hollow area, our forest bird diversity has increased. Fallen trees across our first-order, headwater stream, attract several breeding Louisiana waterthrushes and winter wrens. Barred owls court and nest in our larger, hollow trees. Scarlet tanagers, worm-eating warblers, and red-eyed vireos, among others, are more common than ever. Once we had no nesting black-throated green warblers, cerulean warblers, blue-headed vireos, winter wrens, or Acadian flycatchers, but our aging forest has attracted them. All of these bird species and several more, such as blackburnian warblers — depending on whether species need coniferous, deciduous or mixed deciduous forests — use older, mature forests. And the larger these forests are, the better the chances are for the birds to fledge nestlings.

The same is true for many mammal species. Bears, raccoons, and porcupines, for example, like to den in large, hollow trees. One study of black bear den trees found that in order for red and white oak trees to be big enough, they had to be between 175 and 280 years old, which reminds me of the huge oaks our boys found more than 30 years ago at the steep base of our mountain on a property line with a neighbor. They took some box camera photos of themselves standing in front of them and they looked as if they were as large as California redwood trees. I was amazed and delighted, but before I had a chance to see those trees, our neighbor’s logging operation had cut them down. The boys went to check on them and said that they were all hollow inside, so excellent habitat had been destroyed and not a dollar earned on those trees.

The same den tree study found that raccoons liked tree hollows in trees from 90 to 164 years old, and gray squirrels 65 to 130 years in age. Many bat species, too, like old trees with cavities and loose bark. Other opportunities for denning in older forests include in soil pits created by large root masses of wind-tilted trees, in the root masses themselves, and in stumps, logs, large, horizontal limbs and cavities in standing trees, all of which we have in abundance in our aging forest..

One special kind of older forest that has diminished greatly is that of mixed red spruce and hemlock in the northern tier of Pennsylvania. Such a forest especially appeals to red-backed voles, water shrews, and state-endangered northern flying squirrels. According to researcher Dr. Carolyn Mahan of Penn State, this type of coniferous forest creates a moist microclimate that supports a diversity of fungi, which both the voles and the northern flying squirrels thrive on. They also spread the fungi spores, thereby enriching the soil. Water shrews also seem to prefer such forests, but they like them to be in swampy ravines.

Megalodacne heros beetles feed and mate on varnish shelf fungi, found on an ancient hemlock logs

Megalodacne heros beetles feed and mate on varnish shelf fungi, found on ancient hemlock logs

We also have red-backed voles in our deciduous forest, and Mahan explains that they are not specific to old red spruce/hemlock forests, but more of them are found there than in forests such as ours. Northern flying squirrels are much rarer in our state because they are habitat specialists and their red spruce/hemlock forests have been lost to habitat fragmentation from development of all kinds and to hemlock woolly adelgids. The remaining smaller, patchier coniferous groves next to deciduous forests also attract the more generalist and numerous southern flying squirrels. They are sharing nest sites and even hybridizing with northern flying squirrels, and in the process, passing on a roundworm species, Strongyloides robustus, to which they seem to be immune but which is killing the northern flying squirrels.

Mahan has 600 nest boxes in 21 study sites for the northern flying squirrel, and this year not one northern flying squirrel has been found in any of those boxes. Last year she and other researchers planted an experimental 2500 native red spruce seedlings among the dying hemlocks of a site and others in a recent Game Commission clearcut which was fenced to see if they will grow and thrive and someday produce more red spruce forests. If they do well, they will plant more red spruce seedlings. But think how long it will be before there will be another red spruce forest as magnificent of those we had in Pennsylvania. How much better it would have been if we had saved larger pieces of our older spruce forests.

After talking to Mahan and other researchers, I am more determined than ever to keep growing an older forest. But on this hot summer day, I most appreciate our mature forest for its deep shade that cools not only me but all the creatures large and small that live here.

All photos by Dave Bonta ~ click on images to view larger versions at Flickr

Black-legged Ticks

This marks the 20th anniversary of my column for the Pennsylvania Game News. The first appeared in January 1993 and concerned the Carolina wren. Thanks for reading!

Black-legged tick

Last January I walked along the Black Gum Trail. Since our son, Dave, constructed the trail halfway up Laurel Ridge, back in the 1990s, I had never been able to take the trail in winter. Usually, it was deep in ice and snow as was our north-facing hollow road. But on that mild day there was not a smidgeon of ice or snow on the trail or road.

I neither saw nor heard any creature despite the warm day. The long-promised sun was trying to shine through a matrix of puffy, white clouds drifting past patches of blue sky. At dawn it had been 34 degrees and breezy, and the thermometer had been slowly rising all morning.

Then, as I descended the trail, I glanced down at my pants and socks and pulled off seven adult black-legged ticks. I could hardly believe it. I had considered winter to be tick-free on our mountain. Usually, they spend their winters buried under leaf litter that should be covered with snow. But they are tough creatures, and as soon as it warms up they are out and about. At that time the adult females are not carrying Lyme disease because they had had their last blood feeding on white-tailed deer. Some even winter on the deer.

But, as Dr. Richard S. Ostfeld of the Cary Institute of Ecosystem Studies in Millbrook, New York says, don’t blame deer if you get Lyme disease. The immune system of deer kills the bacteria that cause the disease.

“We don’t know why,” Ostfeld says, “but the deer immune system clears the infection. When they get bit, they wipe out Lyme. Deer play a tremendous role in suppressing adult ticks from spreading the bacteria.” He also dislikes the name “deer tick” and prefers “black-legged tick.”

three deer in snowy woods

White-tailed deer in Plummer’s Hollow (photo by Dave Bonta)

After all, like any arachnid to which ticks are closely related, the nymphs and adult ticks have eight black legs. But the larvae only have six. The larvae hatch from the several hundred to a few thousand eggs each female adult tick lays in spring. She then dies. Both the larvae the first summer and the nymphs the second summer feed once on a mammal and prefer white-footed mice, although they will feed on other small mammals or birds if they can’t find a mouse.

And it is white-footed mice that are the real culprits. They can get the Lyme disease bacteria and pass it on to the ticks even though the bacteria don’t seem to sicken them. Because nymphs are so small, no larger than a poppy seed, they are liable to bite and never be detected during the three to four days they need to take their blood meal. At least 70% of Lyme disease cases are from those nymphs that do not look like the black and reddish-brown adult female ticks. Instead, they have dark heads and bodies that appear to be translucent. Adult male ticks, which don’t feed but will attach to a host when searching for a female to mate with in the fall, are either black or dark brown.

Entomologist Thomas Say named the black-legged tick — Ixodes scapularis — back in 1821. But the first known case of Lyme disease wasn’t identified until 1975 when several children in Lyme, Connecticut were diagnosed with juvenile rheumatoid arthritis. It turned out to be what later was named Lyme disease. In 1982 scientist Willy Burgdorfer isolated the bacterium causing the disease, and it was named in his honor Borrelia burgdorferi.

Scientists also thought that a new species of tick carried the disease and named it Ixodes dammini. It was only later in the 1990s that they realized the tick transmitting the disease had been around and named long ago. But they did recognize that the tick belonged to the family Ixodidae, the so-called hard ticks. They have a hardened plate called a scutum on their idiosoma region, which is a specialized part of a tick’s body that expands to hold its blood meal.

White-footed Deermouse (Peromyscus leucopus)

White-footed Deermouse (Peromyscus leucopus) by J. N. Stuart (Creative Commons BY-NC-ND license)

Like ticks everywhere, the nymphs and adults climb a shrub or blade of grass, hold out their forelimbs, and wait for a victim to brush past. They also lurk on fallen logs, tree trunks, or even on the ground, especially the nymphs which can’t climb as high as the adults. Since they arrived on our mountain, about six years ago, I no longer have the pleasure of sitting on my hot seat on the ground, my back against a tree, watching the life of the forest. They even reach me on our benches unless I pull my feet up on to them.

Ticks have a Haller’s organ on each foreleg with spiny indentation packed with sensors and nerves capable of picking up a breath of carbon dioxide, heat, sweat, or even vibrations from your footsteps. So no bird or mammal can escape their sudden lunge. As I’ve discovered, the small huckleberry shrubs on Laurel Ridge Trail and the grasses of First and Far fields, are ideal “questing” posts for ticks, as well as the underbrush in our forest off the trails where I rarely venture anymore.

Once a tick arrives on its host, it probes around for a soft, bloody site to attack, often in private crevices. Normally, you won’t feel a thing. As David George Haskell writes in The Forest Unseen, “I suspect they charm our nerve endings, taming the cobralike neurons with the hypnotic music of their feet.”

The tick presses its mouthparts into your flesh and saws an opening. Then they lower a barbed tube, called the hypostome, to draw out blood. Because it takes several days to get a full blood meal, it cements itself to your skin with a glue-like material called “attachment cement,” which is why a tick is so difficult to remove.

During the first 24 hours it is attached, it is harmless. But later, when it is full, it takes water from your blood into its gut and spits it back into you, which is when it can transmit Lyme disease or two other diseases — babesiosis and anaplasmosis. The parasite Theileria microti causes babesiosis and Anaplasma phagocytophiolum causes anaplasmosis. As many as 2 to 12% of Lyme disease patients will have anaplasmosis and 2 to 40% babesiosis. This complicates the diagnosis and treatment sometimes because the tick might transmit one or the other or both diseases and not Lyme to a patient. In rural New Jersey, for instance, the Center for Disease Control studied 100 black-legged ticks and discovered that 55 of them had at least one of the three pathogens.

Black-legged tick on human skin.

Black-legged tick by Jerry Kirkhart (CC BY license)

Both babesiosis and anaplasmosis have flu-like symptoms similar to those of Lyme disease but without the telltale bull’s-eye rash. Some folks don’t recognize or even have symptoms of babesiosis, yet they can pass it on to others through donated blood. So far, Pennsylvania seems to be almost free of those two diseases, but they are more prevalent in New York and New Jersey. Unfortunately, it is probably only a matter of time until these diseases increase in the commonwealth.

Last year was supposed to be especially high in Lyme disease cases. That was because in 2010 there was a bumper crop of acorns, followed by 2011 when there were practically none. Dr. Ostfeld, forest ecologist Dr. Charles D. Canham, and colleagues at the Cary Institute first worked out the connection between the amount of acorns and the population size of white-footed mice. In abundant acorn years mice numbers soar but they crash when the acorn crop fails. According to Ostfeld, that leaves a large number of infected ticks looking for hosts. Without the mice, they are after us instead.

At least one hunter friend of ours contracted Lyme disease last June. Although he did get the rash, he never saw the tick. I suspect it was a nymph that bit him. He also listed four places where he could have picked up the tick — turkey-hunting at our place, at a friend’s country property, and on his own country property, or his backyard at the edge of Altoona.

If Ostfeld’s research is right, his backyard was the most likely habitat. In a paper for Conservation Biology Ostfeld and other colleagues entitled “Effect of Forest Fragmentation on Lyme Disease Risk,” they wrote, “Our results suggest that efforts to reduce the risk of Lyme disease should be directed toward decreasing fragmentation of the deciduous forests of the northeastern United States into small patches… The creation of forest fragments of 1-2 hectares should especially be avoided, given that these patches are particularly prone to high densities of white-footed mice, low diversity of vertebrate hosts, and thus higher densities of infected nymphal black-legged ticks.” Given both the size of our forest and the diversity of vertebrate species, we should have less Lyme disease here.

Japanese Barberry berries

Berberis thunbergii – Japanese Barberry berries by Virens (CC BY-NC-ND license)

On the other hand, another study by Tom Worthley and other researchers at the University of Connecticut Forest in Storrs claims that eliminating the invasive Japanese barberry shrubs (Berberis thunbergii) will help control the spread of Lyme disease, anaplasmosis, and babesiosis because white-footed mice favor the barberry’s habitat.

“When we measure the presence of ticks carrying the Lyme spirochete we find 120 infected ticks where barberry is not contained, 40 ticks per acre where barberry is contained, and only 10 infected ticks where there is no barberry,” Worthley says.

Unfortunately, our neighbor’s old 100-acre property that we were able to purchase only after it was poorly logged, is filled with Japanese barberry and other invasives. It’s also moved into the edges of our fields and even into the edge of portions of our older forest. Eliminating all of these bushes will take many manpower hours. But our caretaker hopes to experiment with a few of his own ideas for removing them over the next several years.

In the meantime, I’ll continue to follow most of the suggestions for avoiding tick bites, including super vigilance of my clothes and body, even in winter, when I take my daily walks.

Little Loggers

Last winter I spent more time watching meadow voles beneath our feeders than I did birds. The heavy snowfall in early December provided perfect cover for them and when most of it melted later in the month, the voles’ runways were easy to see. Several voles had nests near our feeders and often their dark gray heads poked out of them to grab a seed or two.

One Saturday afternoon I sat at our bow window watching birds while I listened to the Metropolitan Opera radio broadcast, but I ended up being more interested in meadow vole behavior. On that day, they ventured farther from their nests along their open runways to eat birdseed, and I often mistook them for dark-eyed juncos until they moved. By finding and then focusing my binoculars on a nest entrance, I was able to get excellent views of the plump, beady-eyed creatures sitting there, running along their runways, or feeding with gray squirrels, juncos, and mourning doves. The squirrels chased the birds and each other but didn’t seem to see the voles. Maybe that’s because these nervous little engines of energy move incredibly fast. Once I saw two, one right after the other, dive into a nest entrance.

Well into January, I continued my vole watching. Probably there were more than two but only a couple were out at the same time. They would pick up a seed with their front paws and, sitting on their back haunches, eat it much as a squirrel might (they are, after all, both members of the Order Rodentia). The voles used our discarded Christmas tree, along with the tree sparrows, song sparrows, and juncos, as cover when they ventured out to nibble dried and still-green grasses on the periphery of the feeder area. Juncos startled them whenever they flew in or foraged near their nest entrances, and the voles always darted back into their nests. But they also paid attention to the birds’ frequent alarms, and when the birds flew up in a panic, the voles dashed for cover.

After a month of vole-watching, snow and ice once again sealed them off from the outside world, an ideal situation as far as the voles were concerned because they were safe from many of their enemies, especially avian predators such as hawks, owls, blue jays, and crows. Even many of their larger enemies–foxes, opossums, skunks, and feral house cats–would have found it difficult to break through the thick ice layer that covered the foot of snow on our mountain during much of February.

In the meantime, the voles lived in their surface runways beneath the snow, where their other major predator–weasels–could have chased them down, or in their five-to eight-inch-in- diameter, globular-shaped nests of grasses where they huddled together to conserve energy during the coldest days of winter. Most often, such groups consist of juveniles staying with their mothers although occasionally one or two adult males may join them. They also ate the roots, tubers, leaves, seeds, fruits and grasses they had previously cached above and below ground in preparation for winter.

In late February two fifty-degree days quickly melted the icy snow cover, and once again the meadow voles were visible below the feeders as they ran along their open runways. But even more amazing were the immense number of vole runways that meandered through the dried grasses of First Field like the mazes in children’s magazines and activity books. These patches of torn-up, matted grasses that scrolled themselves across the landscape had been painstakingly constructed by the voles’ sharp teeth as they snipped off any green sprout that surfaced. Slightly wider than a garden hose, their previous under-the-snow passageways were now exposed to the sunlight and the eyes of predators. Their many domed, grassy nests were also open to the outside world.Vole runways did not cover all of First Field. Voles particularly like the thick cover of bluegrass and First Field still harbors pockets of it that were planted decades ago so that was where many of the nests and runways were concentrated. They also like moist areas of dense vegetation, made up primarily of grasses and sedges. Both the lower portion of our once-lawn, a former wetland, and a three-acre wetland at the bottom of First Field above the stream, were crisscrossed by vole runways. Along the runways, occasional piles of little, brownish-green pellets marked the voles’ communal toilets.

By late March, the meadow voles had begun breeding as the promiscuous males competed for the attention of promiscuous females. After a gestation period of 21 days, a female has her first of eight or nine litters in a season. Those litters range in size from one to 11, with an average, in Pennsylvania, of five to seven. She is bred almost immediately after bearing a litter and has a mere three weeks to tend her young, which are born blind, pink, hairless, and helpless, before she has another litter.

At one week, the young are already covered with fur and their eyes are open. At two weeks, they are weaned, and the following week they are on their own. The females of a litter can breed at four weeks of age and the males at five. All this breeding makes the meadow vole the most prolific mammal in Pennsylvania. Without a wide variety of predators, they would quickly overrun their habitat, especially every third or fourth year when their numbers are high. Back in 1924, one captive female, observed by Vernon Bailey, a mammalogist for the United States Biological Survey, produced 17 litters in one year and a daughter from her first litter had 13 litters that same year.

The meadow vole, whose scientific name in 1815 was Mus pennsylvanica (Pennsylvania mouse) for its type locality in meadows below Philadelphia, is now Microtus pennsylvanicus or Pennsylvania small ear, referring to the vole’s tiny ears. Also popularly known as the field or meadow mouse, it is no friend of the white-footed mouse of field and forest. When vole numbers are high, mouse numbers are low which may explain why we had no mice in our old farmhouse last year. Researchers aren’t sure how the voles keep mice out, but they suspect that the much larger and more pugnacious voles may attack and chase any mice they find. Certainly, I frequently observed the voles chasing each other from the birdseed.Both mice and voles are a necessary part of the food chain, supplying endless meals for larger creatures. But do they serve other purposes in the natural world?

Ecologist Richard S. Ostfeld and his associates at the Institute of Ecosystem Studies in Millbrook, New York, have been studying the effect mice and voles have on tree regeneration in old fields. He built nine, one-third of an acre enclosures in old fields and filled them with high (400), medium (175), or low (80) densities of voles per two and a half acres. In each enclosure, he planted tree seedlings of species that colonize old fields in the eastern United States and discovered that the high-density voles killed 95 percent of the seedlings, the medium-density 80 percent, and the low-density 65 percent. They showed a definite preference for red maple, white ash, and the invasive tree-of-heaven and disliked white pine and red oak. Even those seedlings that they didn’t eat, they clipped off near ground level, leaving distinctive, diagonally cut stumps. For some reason, which the scientists haven’t figured out, voles like to keep their homeland free of tree seedlings.

A separate study of white-footed mice found that they only ate tree seeds. Between the mice and voles, establishing a forest in an old field seemed almost impossible.

The next enclosures Ostfeld built were at the boundary between forest and field, since trees usually invade old fields at the edge of the forest. He left the forest end of the enclosures open, figuring that the voles would stay in the field and that mice would move between field and forest. Again he established the same densities of voles as the previous set of enclosures and again they ate the same kind and number of tree seedlings. The mice turned up their noses at those species and instead ate the seeds of red oaks and white pines.

Over the years, Ostfeld found that few tree seedlings of any species survived if vole numbers were high and mice numbers low, but many tree seedlings thrived if mice numbers were high and vole numbers low. As an ecologist, Ostfeld was fascinated by the influence of the “little loggers,” as he calls voles, on the natural world.

“These rodents…play a strong role in preserving attractive vistas and maintaining the open habitats favored by such other wildlife as deer, turkeys, woodcocks, and bluebirds,” he wrote in Natural History magazine. “And meadow voles, by excluding white-footed mice from some habitats, may reduce the risk of Lyme disease, which is carried by ticks that feed off (and are infected by) these mice.”

Could that be why we have not, so far, seen a tick on our mountain? Or why the wetter portions of our field have not been invaded by any tree seedlings in the 32 years we have lived here?

Everything is indeed connected to everything else as more than one ecologist has observed. And unraveling those connections remains a daunting task even for scientists. Our fields, after all, are not the fields that Ostfeld studied and our voles and mice may prefer and dislike different tree species.

The complexities of the natural world continue to fascinate me and I have Never Enough of Nature, as the late, great scientist Lawrence Kilham entitled one of his books. Who would have suspected that meadow voles, in addition to providing food for many predatory birds and mammals, could not only control mice numbers but the regeneration of forests?