Birds use whatever they can get their beaks on to build nests, including cigarette butts. Surprising new research from the Universidad Nacional Autónoma de México shows that instead of giving baby birds a bad case of smoker's cough, the cigarettes in their nests might actually be helping them. The more used-up filters a nest had, the fewer nest-dwelling parasites called it home.
Since nicotine is a natural pesticide, it's likely that trace remains of the chemical in the butts are keeping away the creepy-crawlies. The researchers still don't know if the birds are using butts because they're good insulators, or if they're somehow aware of their anti-parasite properties. Birds in more wild environments have been known to line their nests with strong-smelling, bug-repelling herbs, so it's possible they're instinctively attracted to that special cigarette stink.