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.
Scientists in Mexico City studied the 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.