In December 1958 the Polish embassy in Beijing refused to let the sparrow killers onto its grounds, so the Chinese surrounded it with drummers and kept the drums going for two days until every bird sheltering in the garden had dropped dead from the air, and the diplomats cleared them up with shovels.

That is how thorough it was. The campaign had been running since the spring. Every village, school and factory was turned out with gongs, pans and firecrackers to keep the sparrows flying until their hearts gave out. Nests were torn from eaves, eggs smashed, chicks killed, and children went through the fields with slingshots and were paid by the bird. The dead came in by the basket, strung on wires, piled on carts, and the totals went up the chain as a harvest. Something close to two billion sparrows in two years. The embassy garden was the last place in the capital with birdsong in it, and the drums got that too.

A zoologist called Zhu Xi had told them before it started that a king had tried this and lost. Frederick the Great of Prussia had put a bounty on sparrows in the 1740s to save his grain and his cherries, and the sparrows went, and the caterpillars they had been eating stripped his orchards to the stalk, and the king cancelled the bounty and paid to have sparrows brought back from neighbouring states and let loose. The whole episode was in the library. The ministry had a figure for what a sparrow ate and no figure for what it did, and a man with the second figure was told to sit down.

The locusts arrived on schedule. Then the grain borers. A sparrow takes a few grains from a field in autumn and feeds its chicks nothing but insects all spring, and with the sparrows gone the insects had the rice and the wheat to themselves. The counties with the most sparrows to lose saw their grain fall the furthest, and the rice and wheat that had been holding the country up went down together.

That was 1959, and the state took its grain quota anyway, on the harvest that had been reported. What was left in the villages was not enough, and the famine that followed over the next three years is the largest in human history: tens of millions dead, the estimates running from fifteen million to forty-five, whole counties in Anhui and Sichuan where the living could not bury the dead. The sparrows were one cause among several and the economists who have gone back to the county records put the bird's share at something over two million people, with another four hundred thousand never born. Two million, from a campaign against a thing that weighs an ounce.

By 1960 the ornithologist Tso-hsin Cheng, who had been cutting open dead sparrows and counting the insects in their stomachs, got the bird struck off the list and bed bugs put in its place. There were almost no sparrows left in China to pardon. So the government did what Prussia had done two centuries before: it bought a quarter of a million sparrows from the Soviet Union and shipped them east in crates, to be released into a country that had drummed every one of their relatives out of the sky.

The animal that does the invisible job is always the one on the ledger as a cost. Take the sheep off a chalk down and the orchids are under scrub in twenty years. Fence the cattle off a Welsh hill and the bracken takes it. Empty a savannah of its herds and the grass goes to fire. All of it written down, all of it in the library, all of it available to the next committee with a figure for what a cow eats.

Beijing paid Moscow for the birds in hard currency. The men who had said so in 1958 were still not allowed to say so in 1960.

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