PARIS: Cigarette butts are widely reviled as an urban nuisance but birds in Mexico City see them as a boon, apparently using them to deter parasites from their nests, scientists say.

Local sparrows and finches incorporate smoked cigarette butts in their nests to provide cosy cellulose lining for their chicks and nicotine to ward off mites, they believe.

A team led by Constantino Macias Garcia at the National Autonomous University monitored 57 nests and found that the tally of bugs declined as the number of smoked butts in each nest increased.

Sparrow nests had between none and 38 used butts, with an average of eight per nest, and finch nests between none and 48, with an average of 10.

The behaviour is an intriguing example of how birds adapt to an urban environment, according to the study, which appears on Wednesday in the journal Biology Letters, published by Britain's Royal Society.

In non-urban settings, some birds line their nests with aromatic plants, which are thought to have antiparasitic properties or to stimulate the nestlings' immune system.

Nicotine, a byproduct of the tobacco plant, has also been used in some pest repellents for crops and in controlling poultry parasites.