Squirrel Migrations

When virgin forests covered much of eastern North America, vast armies of gray squirrels (Scirius carolinensis) periodically migrated over the landscape.  They swam across major rivers and swarmed over farmer’s cornfields, eating every last ear of corn.  The very first tax enacted in Ohio is a tribute to their former pestilence–all homesteaders were required to bring 3 squirrel scalps to the tax collector.  The following passage from the book, America as seen by its First Explorers, by John Bakeless illustrates this stunning phenomenom.

In the autumn hundreds of squirrels were sometimes found swimming from one shore to another.  Merriwether Lewis, on his way down the Ohio to pick up his companion, William Clark, in preparation for the Lewis and Clark expedition, found so many of them in the water that he used his trained Newfoundland to plunge in, catch them, and bring them aboard the boat, where they proved ‘when fryed a pleasant food.’

Boatmen near Marietta found the river ‘completely overrun with immense quantities of black and gray squirrels.’  They climbed fearlessly up the oars to rest on the boats, which sometimes had five or six of them aboard at once.  Since about a third of the little animals drowned before they reached the other bank, travel was sometimes unpleasant because of ‘thousands of dead squirrels putrifying on its surface and its shores.’

On land, a hunting party could easily bring in several hundred squirrels at once, and kills of one or two thousand are sometimes reported.  Kentucky riflemen scorned random shooting at these tiny, lively targets.  There was a local Kentucky joke to the effect that a squirrel was inedible unless shot squarely through the left eye.  Some hunters shot squirrels only through the eye and that only when they saw them in the highest treetops.  Really distinguished woodsmen like Daniel Boone refused to shoot the little animals at all.  They preferred  ‘barking off the squirrel,’ that is, putting a bullet into a branch exactly at their feet.  Audubon, for whom Boone gave a demonstration, wrote ‘The whip-like report resounded through the woods, and along the hills in repeated echoes.  Judge of my surprise, when I perceived that the ball had hit the piece of bark immediately beneath the squirrel, and shivered it into splinters, the concussion produced by which had killed the animal and sent it whirling through the air.'”

J.J. Audubon’s painting of what he called the migratory squirrel–Scirius migratorious.  He incorrectly believed it was a unique species, but it was actually the same species as the common gray squirrel.

Massive squirrel migrations were so common during the early 19th century that J.J. Audubon mistakenly believed migrating squirrels were a distinct species.  We now know mass migration is an occasional habit of the gray squirrel.  Mass squirrel migrations stopped occurring following the felling of the old growth timber by the late 19th century, but they still happen on a much smaller scale.  The last recorded squirrel migration occurred in 1998 and took place in Arkansas and some adjacent states.  There was also a squirrel migration in 1968 on the eastern seaboard from Maine to North Carolina.  The squirrels migrate in all different directions, unless they are crossing a major body of water when they all head in the same direction.  The migrations last 4 weeks and always occur in September–a time when food is normally abundant.

Scientists don’t know for sure what caused these massive squirrel migrations, but Van Flyger, a retired scientist, put forth a strong logical hypothesis in 1969.  Recent squirrel migrations occurred in poor mast years that immediately followed good mast years.  During the good mast years when acorns are abundant, gray squirrels produce 2 litters.  Normally, in September, gray squirrels disperse to new ranges because early fall is when food is most abundant and young squirrels have an easy time establishing a new territory.  This is known as the “autumn reshuffle.”  The movement is influenced by the amount of mast the squirrels spend time burying.  They stay in the same territory when they experience the spent time there storing food.  But if there is an excess of young and a shortage of food, the overpopulation of squirrels will just keep dispersing across the landscape.  It’s the normal autumn reshuffle on steroids.  The squirrels keep moving because there is not enough mast for them to spend a long enough time in the same vicinity to establish a territory.  They must have a kind of biological clock that tells them to stop dispersing because they’ve buried enough acorns in 1 area to keep them alive for the rest of the year.

There is an evolutionary advantage to this dispersing habit.  Populations of gray squirrels from different geographical regions come into contact and breed, resulting in healthy genetic recombinations.  Moreover, beneficial mutations are retained.  This dispersal strategy explains why gray squirrels are of a uniform color (with some exceptions) whereas fox squirrels (Scirius niger) are not. (See https://markgelbart.wordpress.com/2012/07/05/colorful-fox-squirrels-were-they-the-more-common-squirrel-in-the-southeast-during-the-pleistocene/)  Fox squirrels do not disperse across the landscape, explaining why this species of squirrel only occurs locally, and why it sports so many different color variations.  I think it also explains why fox squirrels are absent from so many regions in the south.  After forests across the south were clear cut, dispersing gray squirrels were able to recolonize young forests while fox squirrels were only able to occupy small areas of remnant woods, perhaps where lumber operation left some trees standing.

Gray squirrel migrations undoubtedly occurred during the Pleistocene.  The nut burying habit of both gray and fox squirrels necessarily evolved to protect their food supply from foraging megafauna and passenger pigons, both of which could sweep a forest clean of mast.  Red squirrels (Tamiascuirius hudsonicus) also store food but rather than burying the mast, they collect it in large caches within hollow snags.  Squirrels still had to share buried acorns with bears and peccaries, animals that like squirrels, can detect the scent of underground food.  But they did successfully hide food from bison, deer, and pigeons.

On a side note I did see another example of interesting gray squirrel behavior yesterday.  A gray squirrel advancing into my yard while traveling on a tree branch kept waving its tail and pausing.  It was waving its tail to attract the attention of potential predators, then looking around to detect their presence.  The tail-waving is a smart strategy that gives them time to take evasive action, if there is a predator present.  Lately, a pair of red shouldered hawks have been hanging around my yard, and my cat killed a juvenile squirrel last week, so it was understandably cautious.

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3 Responses to “Squirrel Migrations”

  1. James Robert Smith Says:

    When I was a kid living in Atlanta, the city actually experienced a vast squirrel migration. Must have been 1965 or 1966. What happened was that the acorn crop in north Georgia was not good that year and they swarmed southward in a tremendous wave of squirrel flesh. The streets were awash in the carcasses of dead squirrels hit by cars and trucks.

    My job allows me to witness lots of squirrel behavior. I like watching them if I have the time. I have only ever seen one squirrel exhibit overtly aggressive behavior toward humans. I was in a condo complex a few years ago when a group of kids disembarked from a school bus. A squirrel was making his way to a tree when several of the boys headed him off and began to harass the fellow. However, instead of running and dodging, the squirrel thrust his tail high into the air, the hair on his tail spread out, he arched his back, stiffened his legs, and began to jump stiff-legged in a sidewise manner toward the kids. He did not retreat. All the while he was making a really disturbing, deep-throated “churring” sound with its mouth wide open, revealing its incisors. Eventually he was getting so close to the kids (who had all frozen in place) that I figured the squirrel was going to bolt forward and bite one of them.

    So I told the kids to back off, and they finally did. As soon as the boys were about ten feet away, the squirrel relaxed and headed for the nearest tree and went up the trunk. At no point did I see that squirrel exhibit anything that I would describe as fear.

  2. The Super Squirrel | GeorgiaBeforePeople Says:

    […] mast production following a year of heavy mass production that increased squirrel numbers. (See: https://markgelbart.wordpress.com/2012/08/24/squirrel-migrations/ […]

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