Native vs. Introduced

Two kinds of squirrel

California has native tree squirrels and introduced ones, and most people meet only the introduced kind without realizing it.

The natives are the Western Gray Squirrel and the Douglas Squirrel (plus the rarely seen Humboldt's Flying Squirrel), forest animals that were here long before the cities. The newcomers are the Eastern Gray Squirrel and the Fox Squirrel, brought from the east of the Mississippi (from places as far-flung as Minnesota, Louisiana, and the Eastern seaboard), and are now common in California's populated areas.

Eastern Gray and Western Gray are different species

And to clear up the most common mix-up first: the Eastern Gray Squirrel (Sciurus carolinensis) and the Western Gray Squirrel (Sciurus griseus) are different species, not two regional names for the same animal. The squirrel on a city lawn is almost always the introduced one. The natives are out in the trees.

Where you actually meet them

Sort the squirrels by where you're most likely to run into one, from deep wild forest to the middle of the city, and you can see the bigger picture.

Approximate habitat association. The shaded areas show where each species is most often encountered, not the limits of its range.

For actual geographic distribution, see the Range Maps. This chart shows where people are most likely to encounter each species, not where each species can occur.

California's native tree squirrels avoid the cities. The natives cluster at the wild end. The newcomers fill the human end. Here you can see the difference between a forest specialist and an urban generalist.

The squirrels at a glance

The shy natives

The Western Gray Squirrel is a forest specialist. It needs large, connected stretches of tree canopy, and it stays shy of people. It might slip up the far side of a trunk before you've quite registered it. Even typically where it could learn to take a handout, it doesn't initiate it. It never wanted the city life.

We removed the thing it depends on. As development broke the connected woodland into pieces, the canopy that sustains the Western Gray thinned. The newcomers were naturally suited to a fragmented, human-built landscape, and they moved into the gaps. In the developed woods around Santa Cruz, researchers found the Eastern Gray does not merely outlast the native but actively excludes it from usable habitat. Its retreat has several contributing causes, weighted differently from place to place: habitat fragmentation, canopy loss, disease (including notoedric mange), and competition from the introduced squirrels. How much each matters is genuinely debated. The honest summary is that the native is wild by nature, and the newcomers took the ground it gave up.

The Douglas Squirrel complicates the easy picture in a useful way. It is not a shy, vanish-on-sight animal: it's loud, fast, and territorial, and it will scold you from a nearby branch rather than disappear. Where Douglas squirrels live around people in the forest, at campgrounds, cabins, and trailheads, they can grow noticeably trusting. In how it handles people, the Douglas sits in a middle ground between the wary Western Gray and the bold newcomers. But that boldness is a temperament, not an address: the Douglas is tied to conifer forest and does not follow people out of it into lowland suburbs or city parks. It's the clearest proof that being unafraid of people is not the same as being a city animal.

The bold newcomers

The Eastern Gray and the Fox Squirrel are at ease around people and at home in a human-built landscape.

But adaptability is only half of it. People wanted them here. Towns and campuses welcomed squirrels on purpose. They became the live entertainment in the parks. We humans wanted a small, lively piece of nature you could watch from a bench. The newcomers are the far-western end of a deliberate, century-long effort to put nature back into American cities. (That whole story is told in How Squirrels Came West.)

A living bridge to the wild

Here's one way to understand why the bold squirrels took hold so completely: a city is short on wildness, and the squirrel offered a living bridge to it, one that was close, ground-level, and willing to come to your hand in a way most wild animals never will.

There may be something in the way squirrels eat, too. A squirrel sits up, or even stands, and holds its food in its front paws, a posture that looks a little like our own. Dogs and cats, the animals most of us share our homes with, don't eat that way. This is an observation, not a tested finding, but that human-like profile may be part of why people grow so fond of squirrels.

Why a squirrel and not a bird? The trees already had birds, but a bird keeps its distance. The squirrel meets you halfway. (We may also simply recognize something of ourselves in a fellow mammal; that kinship is its own story, told on the evolution page.) There's an irony in it, too: in the early backlash against city squirrels, those very birds were turned against them. Squirrels were accused, often unfairly, of robbing songbird nests. That turn is part of the larger history.

Why the difference matters

Knowing which squirrel you're looking at changes the question from "is this the right squirrel or the wrong one?" to something more useful: which animal is this, and what does it need?

The natives need forest and connected canopy. The newcomers are well established, and most populations are thriving. They are not the same animal living two lives. They are different species with different needs, and good stewardship starts with telling them apart.

Birth control has been tried: at UC Davis, a vaccine stopped Eastern Fox Squirrels from breeding, but it caused abscesses and every squirrel had to be trapped. In the United Kingdom, where the Eastern Gray is invasive, researchers are developing a contraceptive served from feeders that only gray squirrels can open.

Sources: Krause et al. 2014, Journal of Wildlife Management; UK Squirrel Accord, 2025 fertility control update

Common mix-ups