Never thought we'd find an upside to discarded cigarette butts -- but there is one! Scientists in Mexico City have found numerous birds using the nasty butts to line their nests to repel pests and keep themselves warm! Wild birds have long protected their nests from mite invasion by importing chemical-emitting plants, but now birds living in cities seem to have adapted similar behavior, filling their nests with as many as 48 cigarette butts to make use of the repellent properties of tobacco. The nicotine and other chemicals in discarded filters act as a natural pesticide that repels parasitic mites. At the same time, the filtered butts provide useful nest insulation. Scientists studied nests of house sparrows and house finches that each contained, on average, about 10 used cigarette butts.