The Four Pests campaign was launched in 1958 as part of China's Great Leap Forward, a four-year attempt under Mao Zedong to transform the country from an agrarian society into a modern industrialised one. The campaign aimed to rid China of flies, mosquitoes, rats and sparrows. The first three targets attracted no great controversy, but sparrows were singled out because Mao had heard complaints from farmers about the birds eating grain.
Scientists cautioned against the plan. Zhu Xi, a well-known biologist, cited a previous attempt at sparrowcide in Prussia in the 18th century, which had resulted in an outbreak of other pests. Mao did not listen.
The Four Pests campaign was taken up with zeal. People destroyed any sparrow nests they could find and banged pots and pans to scare the birds away from any that were out of reach. Within two years somewhere in the region of 2bn birds had been killed.
Though sparrows eat grain (especially in winter when other food is scarce), they also eat insects such as locusts and rice borers, which likewise attack crops. In summer, such insects make up most of a sparrow's diet. Once the sparrows had gone, pest populations exploded and many parts of the country experienced severe infestations.
The policy of centrally redistributing crops made things even worse. Since the government was convinced that killing sparrows would mean more grain, it felt justified in taking more crops from the places that had exterminated the most birds.
A working paper for the National Bureau of Economic Research by Eyal Frank, an environmental economist at the University of Chicago, and his colleagues estimated that the anti-sparrow campaign by itself accounted for nearly 20% of the fall in crop production during the famine. The researchers used climatic data to calculate how suitable each of China's thousand-plus counties were for sparrows, reasoning that more habitable counties would have had larger sparrow populations and suffered more from their loss. In highly habitable counties, grain output and fertility rates decreased and death rates rose compared with less habitable ones. The authors estimate the campaign killed about 2m people directly and may have prevented another 400,000 from being born.
In 1960, as word reached the upper ranks of the Party that the sparrow massacre had done grave damage, Mao removed sparrows from the Four Pests list and replaced them with bedbugs. By that point sparrows were almost extinct, and China had to import 250,000 from the Soviet Union to try to restore their numbers.
The Great Leap Forward as a whole caused one of the biggest famines ever recorded; somewhere between 15m and 50m people are thought to have died. China in effect abandoned communism in the 1980s and has never again experienced a famine on anything like that scale. The sparrow population eventually recovered and the birds are once again a common sight across the country.
The odds are a million to one against your being one in a million.