Squirrels and possums apparently think being king of the road means piling up the most roadkills for your species. Although possums ordinarily win the I-got-hit-by-a-big-truck award, squirrels seem to be trying to break those records in some areas. Perhaps this is because squirrels share one biological trait with lemmings: They have been reported to reach such high densities in a region that they make mass migrations. Records from more than a century ago report thousands of squirrels traveling overland in the same direction. Some trips ended when they reached a river too wide to swim.

One proposed cause of the phenomenon is that a year with a bumper crop of acorns and other food sources leads to high reproduction success, which translates into overpopulation the next year if natural food production is low. A few recent records exist of smaller mass migrations, and regional fluctuations in nut production can cause squirrels to leave an area in search of more suitable feeding opportunities. These days, in addition to rivers, busy highways are effective stopping points.

Gray squirrels, living in practically every city in the eastern United States where hardwoods are found, thrive where people live. Parks where pigeons, house sparrows and squirrels live off nuts and popcorn provided by tourists seem to be a preferred habitat. According to one source, the highest population density of squirrels ever reported was not from a forest but from Lafayette Square in Washington, D.C. Less obvious tree-dwelling squirrels in North America include fox squirrels, red squirrels and flying squirrels.

Despite their name, gray squirrels are not always gray. They also come in solid black and solid white. When I lived on Michigan State University’s W.K. Kellogg Biological Station, formerly the estate of the Corn Flakes magnate, gray squirrels were common, but almost all of them were solid black. The local explanation was that the long-since-dead Mr. Kellogg had been partial to a black phase of gray squirrel found in small numbers in the area and had the gray individuals eliminated. The black phase is now the prevalent color pattern in the region. One indicator is that most road-killed squirrels in the area are black. White squirrels, usually a result of albinism, are also reported here and there. As with the black squirrels, white ones may become more abundant in a neighborhood than the normal gray phase because humans make special efforts to protect them.

Gray squirrels, whose natural geographic range is North America, now thrive in England, where many view the American gray squirrel as a pest. Having no natural predators in England, a few individuals brought to London successfully colonized. Their sins include eating bark off native trees, destroying shrubbery and generally being a bloody nuisance. The British seem to have no end of pesky colonists. Gray squirrels have also been listed as a nonnative mammal of Australia where they were once introduced. Although they persisted for a while, for some reason they became extinct, and new introductions are no longer welcome. Australians have enough problems with the introduced European rabbit and the giant marine toad from tropical America without having to deal with yet another foreign invader.

Gray squirrels are also indicted as a U.S pest, especially by suburban residents who like to feed birds. As all who put out birdseed know, when gray squirrels are around, they, rather than birds, can become the primary customers. Logically, you decide that as birds can fly and squirrels cannot, a simple solution will be to suspend the bird feeder from a tree limb or mount it on an unclimbable pole. Logically, yes. In reality, no. Squirrels rival monkeys in their jumping and climbing skills. So far, I am aware of no birdfeeder that gray squirrels cannot eventually manage to outwit. Maybe we should keep trying. Or maybe, just maybe, we should simply learn to enjoy the squirrels, which have far more interesting behaviors than most birds anyway.

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