Mexico City has a ton of unique and interesting sights worth seeing, but you know what might be most overlooked? The bird nests. Weird, right? Well the house finches in Mexico City have picked up a bad habit. They've been collecting cigarettes, or at least the used butts from them, and incorporating them into their nests.

Building a house with cigarette butts might not sound like the greatest idea in the world, but apparently these finches have a very good reason for using the stinky butt fibres in their nests. Quirks & Quarks guest host, Anthony Morgan, spoke with Dr. Constantino Macias Garcia, a biologist from the National Autonomous University of Mexico, who has been studying these birds with his colleagues to find out just how clever these birds are.

This interview has been edited for length and clarity.

Anthony Morgan: Why are these birds using cigarette butts in their nests?

Dr. Constantino Macias Garcia: Well the question of why is difficult to answer. But one consequence is that they reduce the number of the ticks, mites, and fleas that infest the nest.

AM: And are they using like the actual cigarette butts themselves?

CMG: They tear apart the paper that covers the butt and then dismember or separate the fibres, which are cotton fibres. These contain all the substances that have been left in the process of filtering the smoke. And we believe that those substances are what they are after.

AM: So they're not using the whole cigarette butts. They're just using the fibres from them. How did you know they were cigarette butt fibres at all?

CMG: Oh it took a while to realize because I've had some students doing a term project on materials that they were bringing to the nest. There was this consistent amount of fibres - they were small, exactly the same size, and they had the smell of tobacco. So eventually the penny dropped and they realized that it was actually the fibres of the cigarette butts. And sometimes they found the cigarette butt not completely dismembered, so they realized what these were.

The birds mostly tear the cigarette fibres apart before incorporating them into their nests, but not always. (Imelda Delgadillo)

AM: How did you figure out what effect these cigarette fibres were having on the nests?

CMG: The first work we did was just measuring the consequence of the nests having the cigarette butts on the number of parasites. So when the nest was empty after the breeding season, we collected the nests and looked at the amount of cigarette butts that had been added to it and then counted the number of fleas and ticks and other ectoparasites. We found that the more we got it both start being used in the construction of the nest, the fewer ectoparasites we collected. So that gave us a clue that, at least, that was the effect of using the butts. But then we still did not know whether that was the reason why the birds were attracted to them.

In order to do that, we had to manipulate the nests and that is what we did. We collected the lining of the nest after the chicks had hatched and then substituted it with one that we fabricated that was made of felt. In that way, we knew that any cigarette fibres that we found subsequently would have been added as a consequence were manipulation. The manipulation consisted of sprinkling on top of these new bedding, some ectoparasites that were either alive or dead. And at the end of the season, again, we collected these artificial beddings and counted both the cigarette fibres and the ectoparasites.

AM: What did you find?

CMG:

We found, funnily enough, exactly what we expected - that the birds had added significantly more cigarette butts in the nests where we had added the live ectoparasites than the other treatments. - Dr. Constantino Macias Garcia

So it was indeed a consequence of our treatment that they were adding the cigarette butts.

AM: That's incredible. So what is it about these cigarette butt fibres that seems to be keeping parasites away?

CMG: We think it is nicotine because this is one substance that plants evolved to fend off herbivores. It is very aromatic. And it is very likely that that is what is acting as a repellent in this particular case.

AM: And just how bad are these parasites for the birds?

CMG: They can be very bad. There are the ones that feed on the feathers, but the really bad ones, are the ones that suck the blood of the chicks or the parents. And often they also transmit endoparasites, like the ones that produce avian malaria, and that can be lethal. So overall, they have negative effects that are so negative that they can completely abandon the breeding colony and forego one year of breeding if there is a big tick infestation in the area.

Dr. Macis Garcia has been studying the nests of house finches. (Constantino Macías Garcia)

AM: Anybody who's ever purchased or even just seen a packet of cigarettes, has seen these terrible labels on the package that describes very serious, harmful effects of the chemicals in cigarettes. Is there any cost to birds to incorporating cigarette butts into their nests?

CMG: We also worry about that. And basically, we looked into the red blood cells, which in birds, they have nuclei. That doesn't happen in mammals, for instance. Now some toxic substances, often what they do is they interfere in the process of cellular division. So when cells divide, the nucleus may not divide properly. And as a consequence, you can see little bits of nucleus being left out or double nuclei or other abnormalities. We counted those abnormalities in little amounts of blood that we took from chicks. We found that there was, again, a correlation between the amount of these abnormalities and the amount of fibres that had been used to build a nest. So we know for certain that there is a damage that is called genotoxic damage.

AM: But I guess on the whole for these birds, it seems like it might be better to use the cigarettes given the threats of parasites than not to?

CMG: Certainly in the short term that is the case. Our measurements in that other experiment showed that they have a larger weight when they fledge, which is a good thing, if they have more fibres in the nest, which means the overall effect is positive. But that is the short term effect. And we don't have any information on what may happen in the long run.

AM: Do you think other bird species might be doing this as well?

CMG: Yes we know actually that they do. The European sparrow, or English sparrow as they call it in Latin America is doing it. We have actually looked into that as well. And we have some evidence other species are also these material to the nests.

AM: So let's say that I want to travel out, go camping and I'm worried about ticks and parasites. Should I be stuffing my pillowcase with cigarette butts?

CMG: Yeah. You could do that. It could be slightly unpleasant, probably. I'd recommend maybe an extract of tobacco instead.

Paper in Journal of Avian Biology: An experimental demonstration that house finches add cigarette butts in response to ectoparasites