Birds are very smart. They have always used aromatic plants like lavender and yarrow to deter certain insects from invading their nests, and in the modern age, they have come up with a repellant which is perhaps more toxic and less fragrant, but nonetheless effective: cigarette butts.
Researchers at the National Autonomous University of Mexico in Mexico City checked out nests of two native birds and discovered that the more cellulose acetate (found in cigarette butts) the nest had, the less parasitic mites it supported. Further experiments revealed that smoked butts were better at controlling mite infestation than un-smoked “fresh” cigarettes because they contain more nicotine. This suggests that when flying around the park gathering up material for their homes birds might be more partial to a high-nicotine Marlboro than say, a Carlton.
It also suggests that throwing your butts into the street is maybe good for the environment, which is very Malcolm Gladwell-esque. Anyway, if you love birds, smoking, and littering, consider switching to a seriously deadly brand.
City birds use cigarette butts to smoke out parasites, nature.com.