Street-wise city birds are keeping their nests warm and pest-free by lining them with cigarette butts, research has shown.
The nicotine and other chemicals in discarded filters act as a natural pesticide that repels parasitic mites. At the same time, the cellulose butts provide useful nest insulation.
Wild birds are known to protect their nests from mite invasion by importing certain chemical-emitting plants.
The new evidence suggests some birds species in the urban jungle have adapted the same behaviour to harness the repellent properties of tobacco.
Scientists in Mexico City studied nests of house sparrows and house finches that each contained, on average, about 10 used cigarette butts. The number of stubbed-out cigarettes incorporated into the nests ranged from none to as many as 48. In both species, nests with larger numbers of butts were significantly less infested by mites.
To test the parasite-repelling effect, the researchers attached cellulose fibres from smoked and non-smoked filters to thermal traps placed in nests. The battery-operated traps attract mites by generating heat. Fewer parasites were drawn to traps laced with nicotine-laden smoked butts.
Dr Constantino Macias Garcia, from the National Autonomous University of Mexico, and his team wrote in the Royal Society journal Biology Letters: "We provide evidence that urban birds incorporate cellulose from smoked cigarette butts into the nest and that this behaviour entails a reduction in the number of nest-dwelling ectoparasites.
"It appears that this effect may be due to the fact that mites are repelled by nicotine, perhaps in conjunction with other substances, because thermal traps laced with cellulose from smoked butts attracted fewer ectoparasites than traps laced with non-smoked cellulose. This novel behaviour observed in urban birds fulfils one of the three conditions necessary to be regarded as self-medication: it is detrimental to parasites."
Nicotine is a natural defence chemical used by the tobacco plant to ward off plant-eating insects, the researchers pointed out. It had been used to protect crops from pests and also to control parasites in poultry.
The scientists said it was possible the anti-mite nest protection was a happy coincidence. Birds might only be lining their nests with discarded butts because they provide good insulation.