Although we love our birds, I have to admit that they can and do become pests at times. The imported birds such as Rock Pigeons, House Sparrows, and European Starlings are good examples, but native blackbirds, crows, and even swallows are given that designation at times. A pest is simply an unwanted plant or animal, such as weeds in our herb garden, deer in our vegetable garden or gulls pooping on the awnings of seaside restaurants. But “pest” is in the eye of the beholder. I frankly don’t mind when sparrows eat my spinach patch or a phoebe builds a mud nest on our stucco house; it’s their habitat as well, is my view. But I understand when flocks of birds descend on a farmer’s crop.
Take the ubiquitous Rock (city) Pigeon. They poop on windowsills, build sloppy nests on rooftops, and thus are generally unwelcome as the mess they leave might also attract other vermin like mice or insects. So now you often see “pigeon spikes” or some similar mechanism installed on buildings to keep the birds from perching. They can spread disease as well, but those diseases are typically uncommon, mild, and not a major health concern.
House Sparrows and starlings can nest in gutters, downspouts, eves, and other building crevices, creating a bit of a nuisance. Cliff Swallows occasionally build colonies of mud nests on school buildings, apartments, or homes and generate a bit of a mess. Northern Flickers often damage wood-sided homes with their pecking.
The serious bird pests, though, are the ones at airports and farms. The flock of geese that caused a jet to land in the Hudson River is a good example. Waterfowl and gulls are typical pests at airports, partly because many airports on built on wetlands or coastal areas. Airport birds are a serious nuisance and billions have been spent making them less so. Farmers of all sorts have fought battles for years trying to keep birds from eating or damaging their rice, peanut, apple, or whatever crop. In the middle ages, kids were sent into the fields during harvest season to serve as bird scarers and there was even a wooden instrument by the same name that was used.
As always though, when we mess with Mother Nature, the results aren’t always predictable. In 1960’s China, a bounty was put on House Sparrows because they damaged a variety of crops. Billions were killed. But because the House Sparrow ate enormous amounts of insects, some of which were crop pests, the years after the House Sparrow eradication were followed by crop failures due to insect damage and widespread famine.