The monkeys are used in an unidentified air force base in northern China , which is situated right on a major migratory route for birds . Birds are a hazard to Chinese planes in the winter . Monkeys are known to attack bird 's nests from tree tops . The monkeys leave a mark on tree branches , discouraging birds from returning .
Birds are a hazard to Chinese planes in the winter . Monkeys are known to attack bird 's nests from tree tops . The monkeys leave a mark on tree branches , discouraging birds from returning .