When the Berlin Wall came down in 1989, Leonard Bernstein gave a concert in Berlin, including Beethoven's Ode to Joy, with the word "Joy" changed to "Freedom" in the lyrics sung.The orchestra were drawn from both East and West Germany, as well as the United Kingdom, France, the Soviet Union, and the United States. Freedom was in the air and it was not just for people. The wall between East and West Germany had also kept a large population of wild pigs within the eastern forests of Brandenburg.The wall's falling down made it easier for the big pigs--a very big one can weigh over 300 pounds--to leave the woodlands and walk into the town. Warm winters and easy access to food have helped the population increase. Now about 10,000 wild pigs walk around Berlin. The nature of the beast has added to the boom ."The pigs are intelligent," says Marc Franusch, a spokesman for the Berlin forestry department. "They learn to use the neighborhoods.They get used to people, dogs, and traffic." The wild pigs tend to travel in small groups and have been found searching rubbish and gardens, feeding their piglets in the shadow of parked cars, and crossing busy roads.On average, the animals are involved in one traffic accident every day. And despite the fact that it's illegal, some Berliners have been known to give the pigs food. Though wild pigs are protected under German law, the city's forestry department is permitted to kill 2,000 of the creatures every year, targeting mostly young adult animals in forests surrounding Berlin. Pigs within city limits are only shot if they make an immediate threat.No humans have yet been seriously wounded by them, but local dogs have been the victims of their tusks. "The forestry department is not aiming to get rid of the pigs," explains Franusch, "but we do have to reduce dangerous situations." The following factors contribute to the boom of wild pigs EXCEPT  _  .
the Berlin Wall adequate food their own nature warm winter