Like most animals adapted to urban living, the house finches in Mexico City make good use of what we humans commonly throw away.
The tiny Carpodacus mexicanus birds take cigarette butts back to their nests, a habit that researchers are now confident is to keep ticks at bay, even if the toxins have a rather nasty side-effect on the growing chicks.
A team of scientists from the National Autonomous University of Mexico followed up on a previous study conducted in 2012 that left them wondering if the finch’s use of the cellulose fibres found in cigarette butts as nesting material was medicinal, or purely for warmth and structure.
Just because the nicotine-stained garbage happened to be keeping the bities at bay didn’t necessarily mean the finches were seeking it out as a parasite repellent.
“One possibility is that birds extract the cellulose fibres from discarded butts simply because they resemble feathers,” the researchers write in their latest paper.
To find more solid evidence, the scientists watched house finches build their nests on the university grounds, and then swapped the fluffy linings in their nests with clean pieces of felt once the chicks had hatched.
The exchange ensured that there were no parasites near the chicks, and the linings were free of cigarette fibres.
On average, the old nest linings had contained around 70 ticks. Using this as a baseline, the researchers added 70 live ticks to 10 of the fresh nest linings.
They then added 10 dead ticks to 10 other nests, and left a further 12 nests tick-free.
To determine whether there was a relationship between parasite loads and the subsequent collection of cigarette fibres, the team then simply weighed the mass of butts in both the old and new linings.
Based on the numbers, it appears as if the discarded butts are deliberately being used to kill parasites such as ticks, with up to 40 percent more cigarette material being added to the live parasite nests than to those containing dead ticks.
Unfortunately repelling the blood-suckers comes at a cost.
In 2014, the researchers found the higher the amounts of nicotine the birds were putting into their nests, the greater the number of chromosomal abnormalities in the chicks.
The lower parasite counts seemed to help more chicks hatch. They also had healthier immune systems, either by being sensitised by the cigarette fibres or simply because they weren’t fighting off as many parasites.
If the chromosomal damage failed to cause problems until the chicks had left the nest and reproduced, the scavenging behaviour would give them more of an advantage, at least in the short term.
More research would be needed to get a clearer picture of the long term effects.
Cigarette butts are among the most common forms of litter, with around two-thirds of the 6 trillion cigarettes smoked worldwide ending up being flicked to the ground, posing a potential hazard for many organisms.
So while a few finches might appreciate it, claiming you’re helping the wildlife still isn’t going to be a valid excuse for not putting your butts in the bin.
This research was published in the Journal of Avian Biology.