The campaign against the ‘Four Pests’ was initiated in 1958 by Mao Zedong. Sparrows were included on the list because they ate seeds, robbing the people of the fruits of their labour. Crowds were mobilized to eradicate the birds, and Chinese citizens took to banging pots and pans or beating drums to scare the birds from landing, forcing them to fly until they fell from the sky in exhaustion. Sparrow nests were torn down, eggs were broken, and nestlings were killed. Sparrows and other birds were shot down from the sky, resulting in the near-extinction of the birds.
By April 1960, Chinese leaders realised that sparrows ate a large amount of insects, and rather than being increased, rice yields after the campaign were substantially decreased. Chairman Mao ordered the end of the campaign against sparrows. However, it was too late. With no sparrows to eat them, locust populations ballooned, swarming the country and compounding the ecological problems. Ecological imbalance is credited with exacerbating the Great Chinese Famine, in which at least 20 million people died of starvation between 1959 and 1961.
One should never use a Simile or Metaphor in haiku, I know, though here it is a comparative. Is the comparison forced? I think not. Sparrows are ubiquitous, but who notices them? However, when they are universally removed from the environment the result is more horrendous that any prediction.