One thing the tour books don't tell you about London is that 2,000 of its residents are foxes , which fled 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 . Several changes have brought wild animals to the concrete jungles . First of all , 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 suburban areas . In addition , conservationists have created urban wildlife shelters . " The number and variety of wild animals in urban areas is increasing , " says Gomez Jones . 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 edges of buildings in the largest U.S. cities to hunt for pigeons . For peregrine falcons , cities are actually safer than rural areas . By 1970 the birds had died out east of the Mississippi because the DDT , a widely used poison to kill insects , had made their eggs too thin to support life . That year , Tom Cade began raising the birds for release in cities , for cities afforded abundant food in the form of pigeons and contained none of the peregrines' natural enemies . " Before they died out , some migrated to cities on their own because they had run out of natural habitats , " Cade says . " To peregrines , buildings are just like natural habitats . " He has released about 30 birds since 1975 in New York , Baltimore , Philadelphia and Norfolk , and of the 20 pairs now living in the East , half are originally born in the city . " A few of the young ones have gotten into trouble by falling down chimneys and crashing into window glass , but overall their adjustment has been successful . " . The passage is mainly concerned with   _   .
wildlife of all kinds returning to large cities to live falcons in New York , Baltimore , Philadelphia , and Norfolk moose running into glass doors of stores foxes returning to London