Answer: Cigarette Butts

Many species of birds have long been observed lining their nests with natural pest deterrents like leaves rich in antiparasitic compounds such as those of the tobacco leaf. When you’re a bird living in an urban area far from the tobacco fields, fresh leaves are a bit hard to come by.

That certainly hasn’t stopped city birds from harnessing the parasite repelling power of tobacco, however. Researchers in Mexico City studying the nests of common house sparrows and house finches found that the birds gathered cigarette butts, pecked them apart, and lined their nests with the cellulose of the cigarette filter. Sampling of various nests as well as tests conducted on nesting boxes constructed by the researchers revealed that the higher the number of used cigarette butts, the lower the level of parasitic mites.

Given the abundance of nesting material in an urban area and the prevalence of cigarette butt cellulose in the nests, it would appear that the birds actively select the butts as a replacement for the natural antiparasitic vegetation they would otherwise use.