The sight of cigarette butts delicately woven into birds' nests sparks an array of reactions, from relief that birds are adapting to urban environments to disgust at the display of human disregard for wildlife. But a new study suggests that some birds may benefit from nesting with cigarette butts. The nicotine lingering in filters may serve as an insecticide, driving parasites away from the nests and the baby birds living within.

As horrifying as the idea of baby birds growing up in a cigarette-filled home sounds, it's not too surprising: the fluffy plastic in cigarette filters makes a fantastic construction material. However, cigarette butts are undoubtably smelly—a fact that has even driven people away from keeping them around.

But birds are actually quite fond of the chemicals found in some smelly plants, otherwise known as aromatics, from which "essential oils" are derived. Aromatic plants produce these chemicals to defend themselves against insects and other animals that would take them for food—but birds have their own use for them. Some nest-building species, including starlings and blue tits, regularly replenish their nests with fresh aromatics, and scientists hypothesize that the birds use these chemicals as parenting tools.

How would plant-derived chemicals help birds raise their chicks? It's possible that the chemicals boost the immune systems or development of the chicks so that they survive better after they leave the nest; this is known as the "drug hypothesis." Alternatively, the "nest protection" hypothesis suggests that the plant chemicals act as insecticides, driving parasites and other harmful insects from the nest.

Nicotine is an insecticide, although we don't often think of it that way. Tobacco plants generate nicotine because it defends against herbivorous beetles that would otherwise devour the plants--which means a smoker's buzz is caused by a plant's chemical defense mechanism. Some remnants of that insecticide remains in cigarette butts left in city streets, which are then transported into bird nests.

The paper authors wanted to know: does this nicotine remnant act as an insecticide when used to construct nests?

First, the researchers from Mexico's Autonomous University of Tlaxcala evaluated the parasites' prediliction for cigarette butts. They set up heat traps to attract ectoparasites (parasites that live outside the body, such as skin or feathers) from 55 nests: in half, the trap was lined with filter fluff from smoked cigarettes, the other, filter fluff from unsmoked cigarettes. Whether the nest held eggs, chicks, or nothing, the unsmoked traps collected more parasites, suggesting that the desire to stay away from smoked cigarette filters outweighed the urge to move towards the heat in the experimental nests.

In the second experiment, the researchers collected 28 house sparrow (Passer domesticus) nests and 29 house finch (Carpodacus mexicanus) nests from Mexico City immediately after the chicks flew out for good. Disassembling the nests in the lab, they then looked for a relationship between the weight of smoked filter fluff and the number of parasites living in the nest. They found that the more butt fluff there was in the nest, the fewer parasites.

Overall, the paper presented convincing evidence that (1) parasites don't like cigarette butts and (2) nests constructed from cigarette butts had fewer parasites.

However, this isn't the final word on the "nest protection" hypothesis quite yet. The major piece of the puzzle the researchers are missing is the health of the birds. Without that information, there is no way to tell if the reduced parasite load in the nests actually provided any benefit. We don't know how many nestlings survived the nests or how healthy they were. For all we know, the nicotine and other chemicals in the cigarette butt fluff were able to drive away parasites by killing off their hosts--the nestlings themselves.

It's also possible that nicotine wasn't the main driver, but one of the other toxic chemicals found in cigarettes, such as hydrogen cyanide, arsenic, or ammonia drove the parasites out of the nest.

If the results do hold, this study is a wonderful example of wildlife adaptation to urbanization--or at least that birds are resourceful and can still follow their noses in urban environments.

Suarez-Rodriguez M., Lopez-Rull I. & Macias Garcia C. (2012). Incorporation of cigarette butts into nests reduces nest ectoparasite load in urban birds: new ingredients for an old recipe?, Biology Letters, 9 (1) 20120931-20120931. DOI: 10.1098/rsbl.2012.0931