Máo was particularly annoyed by the Eurasian Tree Sparrow (Passer montanus, 树麻雀) part of the diet of which was grain. Chinese scientists had calculated that each sparrow consumed 4.5kg of grain each year — and that for every million sparrows killed, there would be food for 60,000 people. Armed with these statistics, Máo launched the Great Sparrow Campaign to address the problem.

[…]

The campaign against the sparrows was finally terminated in late 1959 when the Academy of Sciences leaders highlighted the findings of scientists such as Zhu Xi and Zheng Zuoxin. Zhu and Zheng had autopsied the digestive systems of sparrows and found that three-quarters of the contents were harmful insects and only one-quarter was human food. This showed that sparrows were beneficial for humans.

On this advice from the Chinese Academy of Sciences, Máo declared a complete halt to the Great Sparrow Campaign, replacing sparrows with “bed bugs” in the “Four Pests” campaign. Suddenly sparrows were not just protected but the domestic population was supplemented by imports of sparrows from [Soviet] Russia!

Eventually, after several years of poor crop yields, the situation began to improve. The number of people who starved in the 1958–1961 famine is disputed — and it’s impossible to say how much of the disaster was caused by the extermination of sparrows — but there can be no doubt that this episode is a stark lesson about the unintended consequences of human interference into natural ecosystems.

(Source.)