https://en.wikipedia.org/wiki/Four_Pests_campaign

The Four Pests campaign (Chinese: 除四害; pinyin: Chú Sì Hài) was one of the first campaigns of the Great Leap Forward in Maoist China from 1958 to 1962. Authorities targeted four "pests" for elimination: rats, flies, mosquitoes, and sparrows.

The extermination of sparrows – also known as the Eliminate Sparrows campaign – resulted in severe ecological imbalance, and was one of the causes of the Great Chinese Famine which lasted from 1959 to 1961, with an estimated death toll due to starvation ranging in the tens of millions (15 to 55 million).[note 1] In 1960, the campaign against sparrows ended, and bed bugs replaced them as an official target.

Millions of sparrows were killed.[37] While the campaign was meant to increase yields, concurrent droughts and floods as well as the lacking sparrow population decreased rice yields.[38][39] The extermination of sparrows upset the ecological balance, which subsequently resulted in surging locust and insect populations that destroyed crops due to a lack of a natural predator.[40][41]

With no sparrows to eat them, locust populations ballooned, swarming the country and compounding the ecological problems already caused by the Great Leap Forward, including widespread deforestation and misuse of poisons and pesticides.[39] Although sparrows were removed from the Four Pests in 1960, the disruption of ecological balance, combined with errors in food distribution policies and the exaggeration of crop production figures, led to the Great Chinese Famine.[42][43][44][45] According to a 2025 study, the anti-sparrow campaign accounted for a nearly 20 percent drop in crop production, leading to the deaths of two million people.[46]

By the end of the Four Pests campaign, the Eurasian tree sparrow was practically extinct from China, which afterwards imported 250,000 Eurasian tree sparrows from the Soviet Union to recover its population.[46][47]