Urban Blue Tits and Cigarette Butts: Accidental Litter or Adaptive Behaviour?
Blue tits use cigarette butts in nests to ward off parasites, study finds
While smoking is a well-known carcinogen, what's less appreciated is that scientists have observed some species of birds adding bits of cigarettes to their nests. Scientists in Poland say they have discovered one possible reason why: The cigarettes might be boosting the health of nestlings by warding off parasites.
"Our study indicates that cigarette butts may be used by urban blue tits as an opportunistic adaptive strategy, mimicking the function of aromatic plant materials in parasite control," the scientists write in a paper published earlier this year in Animal Behavior.
Add this to the mountain of evidence about how species are finding clever adaptations to urban life in the Anthropocene. There are glaring examples, such as raccoons turning into suburban "trash pandas." There are more subtle cases, such as bacteria that have evolved to feed on plastic.
Then there's the Eurasian blue tit. The showy little songbird, which resembles a chickadee dipped in blue and yellow paint, is a common site in much of Europe. It's already shown a knack for adapting to human surroundings, frequenting bird feeders and artificial birdhouses. In the 1920s, people in Britain discovered the birds were pecking through the foil covers of milk bottles delivered to doorsteps to feed on the thick layer of cream below.
More recently, scientists have observed a somewhat less wholesome habit. Blue tit nests are found to contain cigarette butts. Some of the birds have even been spotted building nests inside enclosed receptacles designed for people to throw away cigarette butts.
A group of Polish scientists at the University of Łódź, who have a long-term project studying blue tits around that city, decided to try to figure out why these birds might be seeking out bits of trash contaminated with everything from arsenic to heavy metals.
One leading hypothesis was that the birds were using these butts as little pest repellents to ward off parasitic bugs such as mites, ticks and fleas that can infest nests and feed on the blood of young birds. In more natural settings, blue tits are known to add pieces of aromatic plants such as lavender and mint to their nests, and there is some evidence it improves nestling health. Scientists in Mexico have also found that house finches there appear to adorn their nests with cigarette butts to battle parasites such as ticks.
To sleuth out what blue tits were doing, the Polish scientists set up an experiment. They randomly selected 33 birdhouses occupied by blue tits that were part of the long-term research. In 11 of those houses, 5 days after the eggs hatched the scientists added two smoked Marlboro cigarette butts. In 11 others they replaced the nests with sterilized nests fashioned from moss and cotton. In the last 11, they did nothing except add two un-smoked filters from the ends of cigarettes. When the new birds were 13 days old, the scientists took blood samples and body measurements to gauge their health.
The results showed that the cigarette butts were, in some ways, linked with healthier chicks. Young birds in the nests with smoked cigarette ends had higher hemoglobin and hematocrit levels in their blood than the birds in nests with unused filters. Lower levels can be linked to blood loss from parasites. Birds in the artificial, sterilized nests had the highest hemoglobin levels and had hematocrit readings similar to the birds in nests with the smoked cigarettes.
When the scientists scrutinized the nests, however, they found less clear evidence that the cigarettes translated into fewer bugs. There was little difference in the number of fleas, ticks or mites in nests with smoked cigarettes versus unused filters. The number of blowfly larvae was lower with the used butts, although the difference was "marginally" statistically significant. The artificial nests, by contrast, had lower levels of nearly all parasites.
While the bug counts weren't as clearcut as the health checks, the scientists said the results "suggests that even a modest decrease in parasite load, or a shift in the composition of the parasite community, may lead to measurable improvements in chick condition."
What's less clear is whether those benefits outweigh the potential downsides of peppering a nest with an object laced with toxic chemicals. The Mexican researchers have previously reported that while house finch chicks in nests with cigarette bits appeared healthier, their blood cells also bore signs of genetic damage.
How those pros and cons add up could determine whether the blue tit's new cigarette habit is weirdly healthy, or an adaptive dead end.
Gladalski, et. al. "Urban blue tit nests and cigarette butts: accidental litter or adaptive behaviour?" Animal Behavior. Jan. 27, 2026.
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