Big-city ecosystems feature large populations of a very small number of species. Natural ecosystems, outside of a few very difficult environments that offer few niches, tend to move in the opposite direction. The natural tendency reaches its apotheosis in the tropical forest, where rarity is the common condition of a bewildering number of species.
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A typical Chicago neighborhood–a neighborhood of single-family houses mixed with two- and three-flats, with maybe a somewhat larger apartment building on the corner–will have pigeons too, though more will feed there than nest there, along with house sparrows and starlings. The native songbirds will be robins, cardinals, and occasionally black-capped chickadees. Chimney swifts and nighthawks will also be around. If you get really lucky, you may get a pair of kestrels nesting on your block. Kestrels are small falcons that live on a mixed diet of mice, insects, and small birds. They are one of two American raptors that have adjusted to urban living. The other is that much larger falcon, the peregrine. Peregrines did nest on tall buildings in a few cities before DDT eliminated the species east of the Rockies. Now that they have been reintroduced, we have a few birds downtown and along the lakefront. Out in the neighborhoods kestrels are the closest things we have to top predators.
Raccoons and opossums can be added to the mammal list in the neighborhoods. When I first moved to my present neighborhood, I thought the raccoons were all coming to my backyard from the river, which provides a green corridor into the city. Before that I lived just west of Graceland Cemetery and thought the cemetery was a reservoir producing raccoons for the surrounding neighborhoods.
And that is pretty much all there is to the vertebrate side of an urban ecosystem. There are more animals around than most city people notice, but the level of diversity is quite small. We could have as many as 150 species of birds nesting in Cook County, but only about 20 of those might be found in a city neighborhood or most suburban neighborhoods. A Cook County with nothing but neighborhoods, shopping malls, and industrial districts would have 20 to 30 species of nesting birds.
The secret to success as an urban animal is essentially the same as the secret to success for urban humanity: be smart, be adaptable, and be willing to eat anything. It’s hard to imagine a long-tailed weasel eating kung pao chicken, but raccoons and opossums love it.
Some observers of this increase in animal complaints believe the trespassing animals are refugees driven from their homes by development. If they are, then the complaints should drop in the next few years as these animals die off without issue.
