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?