Along with discarded plastic of various kinds—plastic bags, drinking straws, fast-food containers, and the like—cigarette butts are one of the most widespread forms of trash in storm drains and waterways. They’re small enough to pass through many coarse filters, yet collectively they add up to tons of material—as much as 90,000 tons a year in the US, according to one estimate. Some of their components are toxic to aquatic life. They’ve even been singled out for clogging the spaces between pavers and preventing water from infiltrating as planned.

But for all that, they may have a surprising benefit to some species, and their very toxicity is part of it. Researchers in Mexico have demonstrated that certain birds deliberately seek out discarded cigarette butts for nesting material. Nests containing the butts are less likely to have blood-sucking parasites, which can harm newly hatched chicks.

As this article explains, the researchers weren’t sure at first whether city-dwelling birds actively sought the cigarette butts or whether they simply picked them up along with twigs and various man-made materials like electrical cable and aluminum foil, making use of whatever they encountered in the urban environment. So they devised an elaborate experiment, which involved introducing ticks into the nests of house finches whose eggs had just hatched.

After the baby birds had fledged, researchers collected and analyzed the nests’ linings. Nests into which they had placed either dead ticks or nothing at all contained no new cigarette butts. Those into which they’d placed live ticks, though, contained butts that the birds had recently collected, suggesting a pest-control strategy at work.