by William Cocke

With shortening days presaging the coming of winter, Hailee, what you’re probably seeing is an increase in activity by our most common arboreal rodent, the eastern gray squirrel (Sciurus carolinensis). Ubiquitous in urban and suburban settings, this treetop-dwelling squirrel is preparing for the lean times to come with a flurry of food-gathering.

Then again, what you’re observing could be the opening stages of a mass migration. When conditions are right-or more accurately, wrong-gray squirrels will vacate an area en masse in search of food and shelter.

Like many small mammals, squirrel populations are cyclic, fluctuating from high to low depending on the availability of food, habitat, and the effects of predation and disease. Although squirrels are opportunistic feeders, foraging for everything from fungus to inner tree bark, their diet consists mainly of fruits, seeds, and nuts. Oak, hickory, and beech trees supply the acorns and other nuts that make up the staples of their diet. Occasionally, the mast, or nut, crop will fail throughout an area. Suppose that failure occurs after one or more years of good production and, as a result, squirrel populations start to fall dramatically. This combination of factors can result in the mass migration of thousands, even millions, of squirrels looking for new sources of food.

Back when the Eastern forest stretched unbroken from Florida to Maine and west to the Mississippi, squirrel migrations (really one-way emigrations) may have taken place in five- to ten-year intervals. Historically, we’ve had plenty of dependable observers noting this cyclic phenomenon. In September 1803, as he was making his way from Pittsburgh to St. Louis to begin his famous expedition, Meriwether Lewis noted the normally water-averse tree dwellers in the Ohio River: “Observed a number of squirrels swimming the Ohio...they appear to be heading South. Perhaps it may be mast or food which they are in search of, but I would rather suppose that it is climate which is their object.”

Later, in the early 1840s, John James Audubon and John Bachman, wrote of the gray squirrel’s migrations: “It is stimulated either by scarcity of food, or by some other inexplicable instinct, to leave its native haunts and seek for adventures or food.”

Naturally, with the settlement of the continent and the subsequent loss and fragmentation of habitat, squirrel migrations ceased to occur in such mind-boggling numbers. Still, full-blown migrations, though less frequent and on a much smaller scale, have been recorded in recent history.

In September of 1968, reports came in from Florida to Vermont of squirrels on the move in unusual numbers. Of the millions migrating throughout the eastern United States, many died crossing rivers or reservoirs, though most met their end on roads and highways. No one is quite sure why they decided to move over such a wide area. Populations were high after a bumper crop of acorns the year before, yet despite mast failures over much of the range, widespread starvation wasn’t obvious. One theory holds that these periodic population shifts are evolutionally valuable, as squirrels from one area mix with those from another, thus strengthening the genetic diversity of the species.

Whatever the reason, it’s a good excuse to watch these critters closely this time of year. You might just witness as did this 1811 observer: “A countless multitude of squirrels, obeying some great and universal impulse, which none can know but the Spirit that gave them being, [leave] their reckless and gambolling life, and their ancient places of retreat in the north, and [press] forward by tens of thousands in a deep and sober phalanx to the South.”

William Cocke is always turning over rocks and rotten logs to see what’s underneath. He can be reached at wtc4q@virginia.edu.