We never actually established why birds suddenly appear every time you are near. It might just be because you are one of the terrible, horrible people who throws cigarette butts on the ground everywhere. When a little bird waddles out and picks one up and uses it to build a nest, though, you are sort of redeemed, in that the world becomes a better place for its bird family.
Research published today in the journal Biology Letters followed urban birds and measured the amount of cellulose acetate (from cigarette filters) in their nests. The nests with more butts had fewer parasites.
We've known for a while that nicotine is an arthropod repellant. (Are smokers less susceptible to ear mites?) And while we know that birds historically bring certain plants into their nests that will clear our parasites, the biologists are not clear on whether the birds are knowingly employing this effect with the cigarettes. They write, "Urbanization changes the abundance and type of resources upon which birds depend ... Potential changes in host-parasite interactions as a consequence of urbanization may thus influence which species are most able to exploit urban landscapes."