Across Ecuador, Poland, Mexico and New Zealand, birds such as Darwin’s finches, blue tits, house finches and song thrushes have been found collecting cigarette butts and placing them in their nests. Some birds even nest in open ashtrays. Researchers have found that these birds use chemical residues in the filters to keep parasites away from their young. Yet the same substances also affect developing chicks at the cellular level. What seems an odd urban quirk shows how deeply human pollution has penetrated into natural ecosystems. Ironically, the butts kill pests but also imperil the hatchlings. This dilemma is an indictment of human negligence. The story is less about smart birds and more about us.