Birds are keeping their nests warm and pest-free by lining them with cigarette butts, research at St Andrews University has suggested.
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 also known to protect their nests from mite invasion by importing certain chemical-emitting plants.
The new evidence suggests some bird species in the cities have adapted the same behaviour to harness the repellent properties of tobacco.
St Andrews University scientists studied nests of house sparrows and house finches that each contained, on average, about 10 used cigarette butts in Mexico City.
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.
The study was carried out by Dr Constantino Macias Garcia and his team at St Andrews University.
Writing in the Royal Society journal Biology Letters, the team said: "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."
Anti-mite nestNicotine is a natural defence chemical used by the tobacco plant to ward off plant-eating insects, the researchers pointed out.
It has 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.
Further studies could reveal if this is the case by offering the birds a choice of smoked and non-smoked butts.
Either would do for insulation, but only filters from smoked cigarettes can effectively repel mites.
"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 olfaction to collect those with effective chemicals," the scientists wrote.
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