Crows are as common as aircraft in the skies over Chicago. I don’t think anybody has been keeping careful count, but I’m sure there are far more of these big black birds around than there were ten years ago. I see them all over the city, from Michigan Avenue to Cicero Avenue, decorating trees in the park and perching on church steeples.

At Somme Woods in Northbrook–where we have just started to work on this year’s survey of nesting birds–you can’t go five minutes without seeing or hearing at least one crow. I am using large-scale maps of the Somme Forest Preserve to record my observations of the various species that nest there. After only two visits to the place, my crow map is already heavily marked with Xs indicating perching crows and arrows showing the routes of flying crows. By the end of the season I foresee a map that has totally disappeared under a solid layer of Xs and arrows.

I view this crow population explosion with mixed feelings. They are interesting birds: smart, crafty, adaptable. And it is nice to be able to look up into the sky over Clark Street and see a big bird that is not a pigeon. Up in Ravenswood where I live the last resident raven probably expired about the time of the Chicago Fire, so I look on the numerous crows in the neighborhood as low-rent road-show replacements. They are big black birds, but they lack panache. No developer would ever name a subdivision Crowwood, and while crows can be taught to talk, the sight of one sitting on a bust of Pallas and cawing “Nevermore” would produce more laughter than dread.

Rooks–Old World birds of the same genus as crows–have enough status to allow an architect to name an elegant building the Rookery. A structure called the Crowery is impossible to imagine. We might name a penthouse saloon the Crow’s Nest, but that metaphor is really from ships, not the birds.

In our culture crows are generally not eaten, although some people say they are palatable. I’ve never tried one. Once when I was a kid staying on my grandparents’ farm, I shot a starling. Since my grandfather’s rule was “You shoot it, you eat it,” I plucked the bird, singed its pin feathers, and eviscerated it. My grandmother roasted it for me. I honestly have no recollection of how it tasted, which means it probably wasn’t too bad. The British eat rooks, but the fact that the British eat something is not really much of a recommendation.

The omnivorous crows are just the sort of creatures that succeed in the urban–and suburban– ecosystems that now cover most of the Chicago area. They are raccoons with feathers.

Crows have always been thought of as pests. The dummy in your garden is a scarecrow, not a scarerobin or a scaregrackle. A flock of crows can pull up a field full of newly sprouted grain or denude a fruit orchard in a morning. Farmers have been persecuting them for centuries, usually with shotguns, but sometimes with more powerful weapons. There is an account in the ornithological literature of an attack on a large roost in Texas that employed 180 bombs, each powered by a stick of dynamite. The bombs–which showered pellets and shrapnel indiscriminately on everything in the vicinity–were wired together so they could be set off simultaneously. The first blast set off 60 of the bombs and killed an estimated 40,000 crows. The second blast detonated the remaining 120 bombs and killed another 40,000.

You could cite this as a good example of our national fondness for solving problems with bombs, but of course those 180 bombs did not solve anything–although they must have created a terrible mess.

It’s the predatory side of crows that inspires most of my negative feelings about them. They are not primarily predators. Their diets are mainly vegetation, and much of the meat they do eat is scavenged from the carcasses of creatures already dead. But they are known to kill birds, and they commonly go after eggs and nestlings.

I’m not suggesting that there is anything evil about this. As Hobbes (the philosophical tiger, not the philosophical Englishman) recently observed, we are put here on earth to devour each other alive. The thing I worry about is balance. Predators that enjoy eating birds usually get fat in the summer. All those eggs and helpless nestlings and all those clumsy, stupid fledglings are easy pickings. But if the predators get too efficient, they kill off next year’s breeding stock. They bring lean times on themselves, and their own numbers decline. And while July is Fat City, January comes around every year and lots of predators don’t make it through.

But crows, like raccoons, have other resources. When times get hard in the forest preserves, they can turn to raiding Dumpsters. The natural population controls that maintain a balance between predator and prey cannot operate in this situation. The ecological impact of crows and raccoons could become very much like the impact of the house cats that spend the summer killing birds and the winter eating Tender Vittles and sleeping next to the radiator.

Crows also make life difficult for hawks and owls, predators that are subject to natural controls. Crows don’t have the muscle to actually attack a great horned owl or a red-tailed hawk, but they do mob them. They gather in great flocks around the raptors, cawing loudly. Obviously, the noise and commotion prevent the hawk or owl from hunting. But with so many crows around, does mobbing drive hawks and owls from possible nesting sites?

At this point I can’t say for sure that the superabundance of crows is doing damage either to songbirds or to raptors. It would be a difficult thing to prove. Our wild creatures are getting hit from so many sides at once that we may never be able to quantify how much harm is due to individual factors such as pollution, habitat loss, predation, and brood parasitism.

Whether or not we can calculate the exact effects of large numbers of crows on the remaining wild ecosystems in our urbanized region, we should be investigating them. And we should be thinking about the many ecological ramifications of our ceaseless alteration of the environment.

It may take centuries for all the effects of ecological change to work themselves through a system. If we convert an open field to a shopping center or a collection of town houses, we can notice all sorts of immediate changes. There is no place for the red-winged blackbirds that were once abundant. The goldfinches can no longer build their nests in the hawthorns and box elders that were invading the field. Parking lots and lawns provide no homes for the meadow voles, and the northern harrier that once hunted the voles has to move on.

These are changes we can confidently predict. But other changes are less certain and may not show up for decades. The deer that lived in the oak woods next to the field are still there, but they can no longer come out into the field to feed. And the development turns the woods into an island and prevents the deer from moving to another natural area. And so 15 years after the shopping center is built, there are so many deer in the oak woods that they are literally devouring the place.

The crows that nest in the tops of the oaks no longer have a field to feed in, but the shopping center is a bonanza for them. A Dumpster carelessly left open, the remains of a pizza, the carcass of a dog run down by the traffic the shopping center brings, all provide a year-round banquet for the crows. Every year the young grow up fat and sleek. And soon the enlarged flocks are feeding on eggs and nestlings of scarlet tanagers and wood thrushes and other woodland birds. And 20, 30, 40 years after the bulldozers moved in to create the shopping center, the destructive process they started is still rippling through the system.

What I have been saying is speculative. I don’t have proof that things have happened in just this way in any particular situation. But the observed effects of development do provide a lot of support for my scenario. And the changes are still going on. Twenty years from now we may be seeing new species exploding into superabundance or declining toward extinction as a direct result of changes in the landscape that we have already created.