It might surprise you, but most squirrel species found their way to North America long before humans ever showed up. Some squirrels made their way here by crossing vast land bridges during the Ice Age, while people later brought a few types into parks and cities for their own reasons.
Let’s look at how ancient migrations shaped native squirrel populations, and how people in the 1800s and 1900s moved squirrels around for parks, hunting, or just because they liked seeing them. That’s why some squirrels feel like true wild animals, while others seem right at home in city parks.
Natural Arrival of Squirrels in North America
Squirrels arrived in North America way before humans started keeping records. They showed up as small, tree-loving rodents and eventually split into all sorts of species that now live in forests, grasslands, and even cities.
Squirrel Evolution and Ancestry
You can trace today’s squirrels back to ancient rodent relatives from tens of millions of years ago. Scientists say squirrels started evolving during the Eocene and Oligocene periods, when small, nimble rodents picked up traits for climbing trees.
These traits—like sharp front teeth, strong back legs, and those long, bushy tails—helped them thrive up in the branches.
From those early tree hoppers, the Sciuridae family emerged. That family now covers tree squirrels, ground squirrels, and flying squirrels.
Tree squirrels such as the eastern gray squirrel (Sciurus carolinensis) and fox squirrel (Sciurus niger) share skull and tooth features with those ancient ancestors. The American red squirrel (Tamiasciurus hudsonicus) belongs to that same family, though it stayed smaller and a bit more territorial.
Fossils and DNA evidence both back up this family tree. Fossils show squirrel-like creatures by about 35 million years ago. Genetic studies connect living squirrels from different continents and help map out how they branched into today’s species.
Migration via the Bering Land Bridge
Squirrels mainly got to North America by crossing the Bering land bridge from Asia. Changes in sea level during the Miocene and later ice ages opened up that route.
You can picture small, tree-loving rodents moving across when the climate and forests allowed it.
These migrations didn’t happen all at once. Different squirrel lineages crossed at different times, which is why some show up earlier in fossils.
Ground squirrels and tree squirrels made the trip separately, and flying squirrels probably followed forests as they spread across Beringia.
As forests grew, tree squirrels found the perfect habitat to keep moving into new areas.
Archaeological layers and fossil sites in North America show squirrel species popping up after these migration windows. That timing lines up with genetic splits between Eurasian and North American squirrel families.
Diversification of Squirrel Species
Once they arrived, squirrels branched out to fit all sorts of habitats. You’ll see big fox squirrels and eastern gray squirrels in open woods and even in cities.
Smaller species like the American red squirrel stick to conifer forests and love hoarding seeds.
Adaptations made this possible. Tree squirrels got better at climbing and spotting danger. Flying squirrels grew a skin flap that lets them glide from tree to tree.
Ground squirrels became expert burrowers and adapted to life on the prairie.
Climate and food shaped where each species lives. The western gray squirrel hangs out in West Coast forests, while Sciurus carolinensis spread all over the east and into cities.
Now, there are more than 50 squirrel species in North America, each with its own quirks and favorite spots. If you want the deep dive, check out this North American squirrel history.
Human Introduction and Influence on American Squirrels
Humans have moved squirrels into new places, changed what they eat, and reshaped their habitats. Parks, yards, and even old gardens have made some squirrel species super common in towns and suburbs.
Deliberate Introduction of Non-Native Squirrels
In the 1800s, city planners and park designers actually released squirrels into public parks just to make them feel livelier. Philadelphia’s Franklin Square got squirrel releases in the 1840s.
Boston Common and other city parks did the same. Park staff even put out food and nest boxes to help them settle in.
You can still see the results today. Some city squirrel populations trace directly back to those early introductions.
The eastern fox squirrel and eastern gray squirrel spread into neighborhoods after getting their start in parks. Humans ended up picking which squirrel traits—like being bold, friendly, or eating just about anything—would become common.
Impacts on Native Species and Urban Populations
When people brought in non-native squirrels, those newcomers had to compete with local species for nuts and nesting spots.
In some places, the introduced eastern gray or eastern fox squirrel pushed out smaller native squirrels.
You might notice more squirrels in cities where there’s plenty of food and hiding spots.
Urban life changed things a lot. With fewer predators and a steady supply of food from humans, squirrel numbers shot up.
That’s great for spreading seeds, but if too many nuts get eaten instead of buried, it can hurt tree growth.
Your backyard bird feeder or fruit tree can actually help decide which squirrel species thrive nearby.
Famous Squirrel Migrations
Some of the big squirrel “migrations” people talk about are really just range expansions, often helped along by humans or changes in habitat.
In the 19th and early 20th centuries, these moves followed introductions to parks and new suburban plantings.
Central Park, for example, had a stable gray squirrel population by the 1870s thanks to early releases.
Researchers like Vagn Flyger tracked how squirrels moved and spread out over the 20th century. Their work showed that squirrels can quickly colonize new places when trees or city plantings connect green spaces.
That’s why you’ll see squirrels pretty much everywhere in eastern and midwestern America now.
Adaptations and Ecological Roles
Squirrels have learned to thrive in cities by tweaking what they eat, where they nest, and when they’re out and about. Sure, they love acorns and hickory nuts, but they’ll happily raid bird feeders or dig through compost if it’s there.
You’ll probably find their dreys tucked into the forks of tree branches or in nest boxes scattered around parks. These city squirrels tend to get pretty bold around people, and sometimes they shift their schedules just to dodge traffic or neighborhood predators.
Squirrels play a real part in the ecosystem. When they bury nuts and forget about some of them, they end up planting seeds for new trees—kind of by accident, but it works.
Of course, if there are too many squirrels in one spot, they might strip seedlings or outcompete birds for food. So, depending on how many squirrels are around and whether folks put out extra food, your local squirrel crowd could be a big help to trees or a bit of a headache for young plants.
