NESTS are made from things birds find in the environment. For those that live far from human dwellings, twigs and leaves predominate. For those that live in cities, the list of materials is more eclectic. Often it includes plastic bags, paper, aluminium foil, electrical cables and even cigarette butts. Most of these have been assumed to be the result of birds simply making do with what the urban world provides them, but a study just published in Avian Biology by Monserrat Suárez-Rodríguez and Constantino Macías Garcia of the National Autonomous University of Mexico has demonstrated that the cigarette butts are being woven into nests not by accident but by design.

That idea has been around, though never proved, since 2012. This was when Dr Suárez-Rodríguez showed that nests which had butts woven into them were less likely to contain bloodsucking parasites than were nests that did not. What she was unable to show was whether the nest-builders were collecting discarded cigarettes deliberately for their parasite-repelling properties, or whether that parasite protection was an accidental consequence of butts being a reasonably abundant building material.

To discover the truth, Dr Suárez-Rodríguez and Dr Macías Garcia set up an experiment involving house finches that were nesting on their campus. They studied 32 nesting pairs of these birds, waiting for their eggs to hatch. As soon as that happened—a time when, the researchers knew, meddling with the nests would be unlikely to cause the finches to abandon them—they collected the linings of the nests and replaced them with cups of felt that were wreathed with bits of plant material commonly used by the birds. Taking the old linings removed both any parasitic ticks present (since these live in the linings) and any butt material, which also tends to get woven into the material. Once the new linings were in place, the researchers added parasites to some of them. Ten of the nests each had 70 live ticks added, ten had 70 dead ticks added and 12 had no ticks added. They then waited for the chicks to fledge and, once that had happened, collected the linings of the nests for further analysis.

Specifically, they looked at the number of butts which the finches had brought to the nests after the new linings had been fitted. Nests that had had dead ticks or no ticks added along with the new linings contained no butts. In contrast, those nests that had had live ticks added to them contained, on average, one-and-a-quarter butts each. Based upon these findings, Dr Suárez-Rodríguez and Dr Macías Garcia argue that their finches are indeed collecting cigarette butts deliberately, to keep ticks at bay and improve the survival of their young.