It sounds like a surreal urban myth: birds in cities may be deliberately incorporating cigarette ends into their nests to ward off insect parasites. But scientists in Mexico offer compelling evidence that it is true.
They show that an archetypal item of street litter, up there with pizza boxes and burger wrappers as a pavement annoyance, may offer medicinal properties – if you're a bird.
For the nicotine deposited in cigarette filter tips though smoking, which is known as a powerful natural insecticide, is thought to be keeping damaging parasites such as mites at bay. Many birds can be badly affected. Nest infestations can be so bad as to cause nests to fail and chicks to die.
The evidence that birds have found a novel way to combat the problem comes from a team of researchers from the National Autonomous University of Mexico and their study of two urban bird species, the house finch and the house sparrow.
Both often use the cellulose from discarded cigarette ends to line their nests, and when the researchers spent a breeding season closely monitoring nests in Mexico City, they found that the more cellulose there was in a nest, the fewer mites it contained.
Their findings, reported in the current edition of the Royal Society journal, Biology Letters, support the recent discovery that some birds, from swallows to starlings, are capable of "self medication" and often bring green plant material to the nest, especially from aromatic plants, thought in some way to protect nestlings.