Scientists from the National Autonomous University of Mexico noticed the tendency for common urban birds in Mexico City to upholster their nests with cigarette filters, a behaviour that has been reported in New Zealand too—a nest found in Porirua in 2010 was described as “thickly lined”.
In other studies, birds in the wild have been observed to line their nests with fresh vegetation, the aroma or volatile compounds keeping parasites away, making scientists wonder if the nicotine and cellulose acetate present in filters could be a city-slicking adaptation of a natural instinct.
The Mexican scientists examined the nests of house sparrows and finches, measuring the amounts of cellulose acetate (the plastic that the cotton-like filters are made from) in each, along with mites. They found that there were fewer parasites in nests with more filter material.
Another reason for the reduction in parasites may be the nicotine, which saturates a smoked cigarette butt and is a known pesticide. (Nicotine has been sprayed onto greenhouse crops and used to combat lice in poultry farms.) In another experiment, more parasites were found in heat traps containing smoked, nicotine-laden cigarette filters than in fresh filters.
It may even be possible that birds select cigarette butts with the most nicotine when scavenging for the filters. However, the effects of the nicotine (and the myriad carcinogens in cigarettes) on the birds themselves have not yet been quantified.