Why cigarette butts are good for birds

Many birds will use whatever they can get their beaks on to construct their nests — including cigarette butts. Now, new research suggests that there might be a side benefit to using discarded cigarettes for nest materials: it seems to keep the bugs out.

Researchers from Universidad Nacional Autónoma de México studied city birds that were making nests that incorporated the filters that lay scattered on the streets. What they discovered was that nests made of more cellulose acetate (from filters) had fewer nest dwelling parasites. Plus, nicotine traces in the used filters may function as a repellant.

The internet is jam-packed with people suggesting nicotine as the active ingredient in home made insecticide, and these researchers showed that the amount of butt material found in the nests was negatively associated with creepy crawlies getting in. The butts dropped on the streets retain high levels of nicotine, and the researchers experimented to show that its presence in a trap was enough to prevent some insects from getting through.

This immediately raises two questions. First, are birds deliberately incorporating nicotine, or are they just picking up butts because they're readily available, and the right texture and warmth to make good nest lining? The scientists have proposed a way of testing this:

The use of cellulose from cigarette butts may be due to other properties of the cellulose (e.g. as a thermal insulator) unrelated to the effect of nicotine on ectoparasites. Presumably, both new and smoked butts can provide thermal insulation, but only the latter would protect against ectoparasites, thus a choice test under controlled conditions could be used to disentangle which is the primary function of this behaviour.

Another question is whether the presence of nicotine (and other chemicals) in the nest might have any health effects on the chicks. While cigarette butts may only contain trace amounts of nicotine, for something as small as a baby bird, that could be a potentially hazardous amount of the stuff.