In his book Mutual Aid a Factor in Evolution, the 19th century naturalist Peter Kropotkin remarks on the flocks of mixed species of birds he encountered in Sibe

In his book Mutual Aid a Factor in Evolution, the 19th century naturalist Peter Kropotkin remarks on the flocks of mixed species of birds he encountered in Siberia. He documented clear cases of inter-species coöperation, like warning of and mobbing predators. It was one of the experiences that led him to his hypothesis that coöperation was a survival trait that would be preserved and spread by natural selection.