Birds in Mexico City Have Taken Up Cigarettes to Protect Their Young

Urbanization has all sorts of side effects. Birds have been shown to adapt their calls, their nesting places and activity rhythms to fit the urban environments. In Mexico City, urbanization is forcing birds to use what they can find to help build their nests and keep parasites away.

One abundant material, the lowly cigarette butt, is showing a surprising benefit.

Cigarette butts are the most common form of litter. It is estimated that two thirds of the 6 trillion cigarettes smoked worldwide end up on the ground and pose a threat to smaller organisms. Scientists in 2012 first conducted a study in which they found that urban birds, mainly house finches, were using cigarette butts to help build their nests.

It is known by scientists and outdoorsmen that nicotine in tobacco can be used as a pesticide. The researchers from the National Autonomous University of Mexico, were not sure whether birds were intentionally using the butts as a pesticide or if it was simply a coincidence since the material was abundant in their urban environment.  

Either way, it's working.

"One possibility is that birds extract the cellulose fibres from discarded butts simply because they resemble feathers," the researchers theorized.

To test this theory, the researchers watched several house finches build their nests. Once the chicks had hatched, the team replaced the used cigarette butts with felt. In describing their process for testing whether the cigarette butts were actually being used to ward off parasites, the researchers went through a simple process. 

"We removed the bedding of nests when chicks had recently hatched, and randomly assigned each nests to one of the following treatments: 1) addition of live ticks, 2) addition of dead ticks and 3) simulation of tick addition. Females in the live ticks' treatment added more butt fibres to their nests than parents in control treatments," the researchers reported.   

What they found was stunning: Birds added cigarette butts to their linings once they saw that they were being infested with a parasite. This showed a deliberate attempt by the birds to use cigarettes as insect repellent. 

The improvised insect repellent came with a cost. In 2014 another study was done to see what affect the presence of cigarette butts does to the chicks of the finch. The results were predictable.

"We found that hatching and fledging success and chick immune response were all positively correlated to the proportion of the nest that was made up of butts. However, the signs of genotoxicity in the blood cells also increased with the proportion of butt cellulose in the nests."   

While the lower parasite counts were beneficial in helping the chicks hatch, and they also had healthier immune systems, however they showed a large percentage of chromosomal abnormalities. These abnormalities did not show until they left the nest and reproduced. The scavenging practice showed that it helped the birds in the short term but that it was detrimental in the long term. 

Throwing your cigarette butts on the ground is not helping the house finch or any other animal life in either the urban or rural environment.