Researchers Suárez-Rodríguez et al., observed house finches (C. mexicanus) and sparrows (P. domesticus) on the National University of Mexico campus in Mexico City routinely pulling apart cigarette butts to obtain the filters. The birds were found to incorporate the filters in their nests leaving researchers to speculate the purpose of used cigarette butts. Using heating elements to attract parasites, they found that probes covered in “used”, i.e. artificially smoked, cigarette butts attracted fewer ectoparasites than those that were not used. These observations suggest that the birds may be using discarded cigarette butts, which collect chemicals like nicotine, for pest control in their nests. Direct examination of the nests post-breeding supported these findings by showing that nests with more butts had fewer parasites.
Since eggs are rather porous, the potential health risks for cigarette butt-insulation on developing embryos or adult birds are currently unknown.
Suárez-Rodríguez M, López-Rull I, and Macías Garcia C. Incorporation of cigarette butts into nests reduces nest ectoparasite load in urban birds: new ingredients for an old recipe? Biol letters.9(1), 2013. doi: 10.1098/rsbl.2012.0931