Came up while I was walking the elm line at the north gate, counting nothing much. A biologist in Michigan had his students picking dead robins off a university campus after the elms were sprayed, and Carson got letters from people who'd simply stopped hearing birds in their own gardens. She put a long list of sources at the back of Silent Spring in 1962, which is why the arguing that followed had to be about the chemicals rather than about her.
The title's Keats, from La Belle Dame sans Merci — "The sedge has withered from the lake, / And no birds sing" — and she put it right at the front. The first chapter's a fable, a town that falls quiet, before a single citation shows up.
The part the chemistry made unarguable: DDT dissolves in fat, not water, and it breaks down slowly, so it doesn't wash out of an animal — it sits in the tissue. Each step up the chain eats a lot of the step below, which means the dose concentrates. A worm carries a little, a robin eating worms all spring carries a lot.