Birds in cities are insulating their nests with cigarette butts, a practice that might help smoke out parasitic mites thanks to the nicotine found inside the discarded material, researchers in Mexico have found.

Scientists at the National Autonomous University of Mexico in Mexico City examined the nests of two bird species common in North America. They measured the amount of cellulose acetate, a component of the paper filters at the end of cigarettes, found in the nests and discovered the more there was, the fewer parasitic mites were present.

Nicotine is a natural defence used by the tobacco plant to ward off plant-eating insects. (Daniel Munoz/Reuters)

Monserrat Suarez-Rodriguez, the lead author of the study published in Biology Letters, says city birds may just be adapting a behaviour that is common in the wild to an urban environment. Birds in the wild are known to line their nests with vegetation that keeps parasites away.

Suarez-Rodriguez and her colleagues went even further in their research and investigated whether there was a difference between material from smoked versus unsmoked cigarettes.

Nicotine an effective pesticide

The team put traps in the nests of 27 house sparrows and 28 house finches on their university campus. The traps were outfitted with fibres and filters from smoked or unsmoked cigarettes and emanated heat to attract parasites.

They found that the traps with butts from unsmoked cigarettes had many more parasites than the devices with butts from smoked cigarettes – which contain more nicotine. The nests with unsmoked butts caught more than twice as many parasites, on average.

"It appears that … mites are repelled by nicotine, perhaps in conjunction with other substances," the study authors concluded.

Nicotine is a natural defence used by the tobacco plant to ward off plant-eating insects and has been used to protect crops from pests as well as to control parasites in poultry.

But the scientists admitted that in the case of the birds, the pest-repelling effect could just be an added benefit of using cigarette butts to line their nests. The discarded butts provide good insulation regardless.

The researchers said they'd like to expand their study to see whether birds are intentionally choosing filters from smoked cigarettes.

"Birds could distinguish smoked and non-smoked butts from their scent, just as some birds that use the chemical compounds of plants as defence against parasites appear to rely on [smell] to collect those with effective chemicals," they wrote.