Six years ago a team revealed that the nests of birds that had cigarettes woven into them were less likely to contain blood sucking parasites than the nests of birds that did not have cigarettes in them. Further testing revealed that the nicotine content in the butts played a key part in keeping the parasites away. I wrote that up in Nature at the time and pointed out that the team behind the research did not really know if city birds were doing this by accident or had actually worked out that cigarettes were a valuable tool for use when parasites came to their nests. Now that same team is revealing evidence that the birds know perfectly well what they are doing.
Precisely why birds bring cigarettes to their nests is a matter of debate. Some argue that they collect discarded butts simply because they resemble natural materials like twigs while others argue that they know the butts repel parasites. To test out this latter idea, the team monitored house finch behaviour as they added either dead or living parasites to their nests. When live parasite numbers increased, female finches collected more butts for their nests than females that had dead parasites added. You can read more in The Economist article that I wrote on this here. Alternatively, if you would like to hear me describe the research on The Economist's science podcast, you can do so here.