There are too many feral chickens (Gallus gallus) in Key West, Florida, according to this article from The Miami Herald. So many, in fact, that the city has hired someone to remove about half of the 2000 hens and roosters that wander the streets, making nuisances of themselves by destroying gardens and crowing at all hours of the day and night. The chickens, while not native, are not having much of an impact on the natural habitat (as I've been informed by more than one person, Key West has virtually no hatural habitat). Whether you're a fan of chickens or a hater of fowl, you'll want to check out the Key West Chickens website.
Update: An anonymous reader has sent along this thought-provoking description of the feral chicken situation in Key West, and has kindly allowed me to post it:
I saw your inquiry about the chickens in Key West. Believe me, it is not a environmental issue, Key West is almost entirely paved and has little left nature to protect. The residential neighborhoods in Key West are tightly packed with homes just 10 or 20 feet apart, and sometimes closer than that. you really get to know your neighbors - and their pets. As far as chickens go, the typical majority of residents have remained silent, though I suspect that most of them find the birds a mixed blessing: sometimes fun to watch, but sometimes messy, occasionally destructive (especially in newly planted gardens), and of course sometimes noisy. They fight, almost like cats (noisy and sometimes bloody), and I have found more than one mortally wounded or blinded rooster dying a slow death under my landscape shrubs as a result of territorial disputes. Worst of all, they are prone to keeping unfortunate hours. Speaking from experience, there are few things as aggravating as being repeatedly woken by one or more roosters crowing off and on through the night and hours before you need to be up, when your neighbor, who lives shut up inside an air-conditioned and relatively sound-proof home, feeds the birds but denies ownership responsibility. The roosters at least have no concept of time, and every porch light or bathroom window that lights up after the dark apparently can be mistaken for a rising sun. Cock-a-doodle-do, indeed. A rising moon causes the same effect and a full moon can keep them going all night. Well-acclimated Keys residents, and those who cannot, for health reasons, deal with air-conditioning, prefer to (or need to) sleep with the windows open and rely on neighbors remaining resonably quiet between 11PM and 6AM. However, even afternoon naps are not safe from these compulsive feathered alarm clocks. The real issue eventually boils down to property rights and zoning laws. Chickens are ruled by Monroe County and Key West city ordinances as "farm animals" and "prohibited in residential-zoned neighborhoods." Therefore, their boosters cannot legally (or practically) confine the birds to their own property, as that would establish ownership of animals prohibited by the zoning law. Instead they deny ownership but put out grain and water, and even replace birds that are removed. But, their chickens have the run of the neighborhood (imposing on the silent majority), causing property damage, leaving eggs (the numbers are shocking) to rot and stink in hidden corners, and even causing a car accident now and then. And of course, waking folks up long before dawn. As with every issue, there are over-the-top extremists on both sides: anti-chicken zealots have resorted to poisoned grain, and chicken boosters smuggle in new chickens to replace those that die or that the city has removed (at tax-payers' expense). Hope I have provided so enlightenment, or at least something to think about.