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The artificial concrete and steel ecosystems of cities, including occasional small, manicured parks, can support a surprisingly large number of birds. The birds are often most visible where people have congregated to buy, sell, eat, and discard food. The commensal relationship (whereby birds vacuum up bits of food that people do not value for themselves — items dropped, discarded, or purposely provided) is evident near benches in city parks, in playgrounds, sports stadiums, and the parking lots of fast-food restaurants, at deserted farmers’ markets and fairgrounds, around refuse disposal areas, and at window ledge feeders.
Urban birds differ from wild populations in several ways, besides just being easier to observe. A number of ornithologists have looked at the ways birds have adapted to urban foraging. For example, in cities that lie in the snowbelt, birds may seek underground heating ducts over which plants can grow, and have also learned to feed in areas illuminated by artificial light where they can prolong their foraging schedule. In another case, although feral (country) pigeons usually eat twice a day, filling their crops at each session and digesting the food between the feeding bouts, city pigeons face a less predictable food supply, and are much more opportunistic, having a relatively irregular feeding schedule.