Adaptation to human-modified environments such as cities is poised to be a major component of natural history in the foreseeable future. Birds have been shown to adapt their vocalizations, use of nesting places and activity rhythms to the urban environments, and we have previously reported that some species, including the house finch (Carpodacus mexicanus), use cellulose from smoked cigarette butts as lining material and thus reduce the number of ectoparasites in their nests, probably because the nicotine repels arthropods. Nicotine is only one of hundreds of potentially harmful substances found in cigarette butts. Here, we investigated whether the presence of such chemicals is harmful for house finches adding cigarette butts to their nests. We found that hatching and fledging success and chick immune response were all positively correlated to the proportion of the nest that was made up of butts. However, the signs of genotoxicity in the blood cells also increased with the proportion of butt cellulose in the nests. Although we have not measured the effect of genotoxicity on post-fledging survival and breeding success, it seems that bringing cigarette butts to the nest has negative consequences that may counterbalance the benefits of using them as ectoparasites repellents.