Urban birds are incorporating cigarette butts into their nests for bug-killing action and for insulation, a new study finds.

The discovery, published in the latest Royal Society Biology Letters, helps to explain why so many avian nests include cigarette butts in their construction.

Isabel López-Rull of the Instituto de Ecología, UNAM, and colleagues came to the determination after studying the effect cigarette butts have on parasites. In short, if you’re a parasite, you don’t want to be anywhere near a cig butt.

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“The amount of cellulose acetate from butts in nests of two widely

distributed urban birds was negatively associated with the number of

nest-dwelling parasites,” the authors wrote. “Moreover, when parasites were attracted to heat traps containing smoked or non-smoked cigarette butts, fewer parasites

reached the former, presumably due to the presence of nicotine.”

The cigarette butts were also found to insulate nests, making conditions cozier for the studied birds: the house finch and the house sparrow. Other birds likely use cigarette butts too, but these happen to be the two species selected for the study.

“Birds are known to respond to nest-dwelling parasites by altering behaviors,” the authors added. “Some bird species, for example, bring fresh plants to the nest, which contain volatile compounds that repel parasites.”

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The usage of cigarette butts then falls in line with this already evolved behavior.

“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 explained.

Image: Passer domesticus in Mexico City. Credit: Victor Argaez