Feral pigeons are often considered a pest or even as vermin, owing to concerns that they spread disease. Also concerns of them damaging property, causing pollution with their excrement, and driving out other bird species. Some also consider pigeons an invasive species. Long-term reduction of feral pigeon populations can be achieved by restricting food supply, which in turn involves litter (garbage) control. Pigeons can breed very quickly — up to six times a year — and their numbers are determined by how much food is available; that is, they breed more often when more food is provided to them.