One thing the tour books don't tell you about London is that 2,000 of its residents are foxes. They ran away from the city about centuries ago after developers and pollution moved in. But now that the environment is cleaner, the foxes have come home, one of the many wild animals that have moved into urban areas around the world. "The number and variety of wild animals in urban areas is increasing," says Gomer Jones, president of the National Institute for Urban Wildlife, in Columbia, Maryland. A survey of the wildlife in New York's Central Park last year _ the species of mammals, including muskrats, shrews and flying squirrels. A similar survey conducted in the 1890s counted only five species. One of the country's largest populations of raccoons   now lives in Washington D.C., and moose   are regularly seen wandering into Maine towns. Peregrine falcons  dive from the window ledges of buildings in the largest U.S. cities to prey on   pigeons. Several changes have brought wild animals to the cities. Foremost is that air and water quality in many cities has improved as a result of the 1970s pollution-control efforts. Meanwhile, rural areas have been built up, leaving many animals on the edges of suburbs. In addition, urban wildlife refuges  have been created. The Greater London Council last year spentPS750,000 to buy land and build 10 permanent wildlife refuges in the city. Over 1,000 volunteers have donated money and cleared rubble from derelict lots. One evening last year a fox was seen on Westminster Bridge looking up at Big Ben. For peregrine falcons, cities are actually safer than rural cliff dwellings  . By 1970 the birds had died out east of the Mississippi because the DDT had made their eggs too thin to support life. That year, scientist Tom Cade of Cornell University began raising the birds for release in cities, for cities afforded abundant food. Cities can attract wild animals without turning them harmful. The trick is to create habitats where they can be self-sufficient but still be seen and appreciated. Such habitats can even be functional. In San Francisco, the local government is testing different kinds of rainwater control basins to see not only which ones retain   the cleanest water but which will attract the most birds. What is the passage mainly about?
Wildlife returning to large cities Foxes returning to London Wild animals living in zoos A survey of wildlife in New York