Last week an old nest lined with old cigarette butts and other types of litter blew down from the branches of our cedar tree. I have noticed cigarettes before in other nests, and had always assumed the birds chose them was because the fibers were warm and soft and easy for the birds to find in the city. But a new study, published by the journal Biology Letters, shows that there may be a chemical reason for the birds to choose old cigarettes as a nesting material. Nicotine, left on the filters after smoking, may be deterring parasites. The birds, it seems, are self-medicating.
To test this theory, researchers working in Mexico City compared the mite numbers at nests made with used cigarettes with those made from unused, unsmoked cigarettes. Although both kinds of fibers can provide thermal warmth to the nest occupants, the researchers found that there were fewer parasites on the nests with the used cigarettes. They also found that a majority of the nests in their study included the butts; 86 percent of the house finch nests and 89 percent of the house sparrow nests included in the study contained them. In addition, the nests with the largest amount of those fibers contained the lowest number of parasites.
Tobacco is actually a native plant in the Americas, and the nicotine it contains has long been used as an insect repellant for other crops. It has even been used by poultry companies to control parasites.
Although the chemical might be helping control mites in urban bird nests, the fibers of used cigarettes are laced with numerous other toxins which the birds are also exposed to during nesting, some of which come from pesticides used during the growing of the tobacco. This nest exposure to those toxins may counterbalance any benefits the birds gain from the use of the butts, the authors say.
“Because urbanization changes the abundance and type of resources upon which birds depend, including nesting materials and plants involved in self-medication, our results are consistent with the view that urbanization imposes new challenges on birds that are dealt with using adaptations evolved elsewhere,” the authors write.
You can find the study on the Biology Letters website:
Incorporation of cigarette butts into nests reduces nest ectoparasite load in urban birds: new ingredients for an old recipe?Biol. Lett. February 23, 2013 9 1 20120931; doi:10.1098/rsbl.2012.09311744-957X
Authors: Monserrat Suárez-Rodríguez, Isabel López-Rull, and Constantino Macías Garcia