The butts are undoubtedly smelly. But birds are actually quite fond of smelly chemicals, such as those found in aromatic(芳香的) plants. Some nest-building species regularly fill their nests with fresh aromatics, possibly because te chemicals are good for the immune system or the development of the chicks. Alternatively, the plant chemicals might act as insecticides.

　　In the study, researchers at the Autonomous University of Tlaxcala in Mexico set up heat traps, which attract harmful insects, in 55 nests around Mexico City. Some traps were lined with the stuff in smoked cigarette filters. The others were lined with the stuff from unsmoked cigarette filters, which did not contain nicotine and other smoking by-products. Whether the nest held eggs, chicks or nothing, the unsmoked cigarette traps collected more harmful insects, suggesting that it is the chemicals that drove harmful insects away.

　　In a second experiment, the researchers collected 28 house sparrow nests and 29 house finch( 朱雀 ) nests from Mexico City immediately after the chicks flew out for food. They found that the more smoked filter stuff filled a nest, the fewer harmful insects it had.

　　The missing piece of the puzzle is whether the reduced number of harmful insects in the nests actually provided any benefit to the chicks. It is also unclear if nicotine or another chemical found in cigarettes may have turned the harmful insects out of the nests.

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