In the early 2010s, researchers in Mexico City noticed that sparrows and finches at the national university were lacing their nests with cigarette butts. The birds would collect the butts—mostly smoked—carefully remove the outer paper layer, and weave fibers from the filters into their homes, among the twigs and grass.
This sort of dubious yet intriguing lifestyle choice will be familiar to anyone who follows health trends. It seems weird—but does it make some kind of backward sense? In this case, the birds were vindicated: The more cigarette filter fibers the nests had, the fewer parasites they harbored, probably because nicotine repels bugs. There are drawbacks, though: Chicks raised in