Pigeons have adapted well to urban environments. Most cities are full of flocks of these birds, just looking for handouts. They often associate humans with food, and are unafraid to flock closely to people. They are also unafraid to leave droppings everywhere (cars are obviously preferred spots), along with nesting material. The nesting material can clog vents and drain pipes, and cause potential fire hazards. The droppings and feathers can contaminate vents of buildings, as well as food destined for human consumption. Pigeons can carry or transmit numerous diseases. Pigeons also contain parasites including a number of bugs, fleas, ticks and mites.