Listen to part of a lecture in an ecology class.

If I ask you to picture a squirrel or a pigeon in its natural habitat, what might come to mind is a city, right? But think about that for a second. Pigeons have been around for much longer than the modern city, so it can't be their original habitat. Before big cities even existed, they lived in the natural environment for thousands of years. Then why do they seem so comfortable, so natural living in an urban environment? Well, many animals have actually changed in order to live in these kinds of habitats, and it's a process known as synurbization.

Now, synurbization basically refers to the adjustments that animals make to survive in urban environments. And many birds and mammals like the pigeon and squirrel have synurbic properties that allow them to make the city their home. As you can imagine, living in the city requires some major changes in an animal's behavior, from its diet to the way it makes its home. To, uh, to go off on a quick tangent here, there's actually still a question about exactly how this works. It's possible that we might not be talking about simple behavioral changes. For example, we've actually found some pretty significant anatomical differences between rural and urban populations of the same species of mouse, and some researchers believe those differences in the mice are actually genetic. So it's possible that, in some cases, synurbic populations are in the process of becoming a different species or subspecies, but those anatomical differences could also just reflect a natural range of variation in a species, like there's a natural range in height among humans.

To get back to what we do know there are a couple of characteristic changes you see in synurbic animals. So for example, animals in cities get comfortable living much closer to each other because, you know, there's not as much open space, so each animal sort of has to give up its private territory and share with other members of the species. Also, you see a big decrease in migration. Winters in the city are generally milder. There are warm places all around, so often animals that would normally migrate can just stay put. There's no reason to travel long distances to find warmer climates. Not only that, but there's also a lot of food in the city that you couldn't find in the wild during a winter, so nutrition is easier to come by. This makes migration even less important.

There are also behavioral changes when it comes to animals’ attitudes toward people. If you've ever seen somebody feeding breadcrumbs to pigeons in the park, you've witnessed synurbization at work. Pigeons and other animals as well have become much more comfortable around humans because they know that's where the food comes from. So for some animals, the city can actually be a really hospitable place, maybe even crucial for the survival of the species. For instance, take the peregrine falcon, an absolutely beautiful bird. A few decades ago, it was nearly extinct, but researchers thought it might survive well in the city, and they started pushing conservation efforts to bring Falcons into urban areas, and now peregrine falcons are absolutely thriving in those cities. They're totally synurbic, and it's thanks to the modern city that they're not a threatened species anymore. The peregrine falcon is living proof that it's possible for cities to coexist with nature in some ways, but that certainly doesn't mean there's no ecological downside to urban expansion. When we see successful examples of synurbization, there's a very real danger that people might misinterpret them.

People might look at those examples and think animals in general can simply adjust to the growth of cities. But as far as species diversity, the expansion of urban areas poses a serious problem. Overall, you end up with a big decrease in the variety of species in an area that has become urban. One study found that 47 bird species either decreased in population or vanished completely from a particular city. So clearly, not every species is equally good at adapting to the city. I think it's fair to say that most species are definitely threatened by urban growth.