When you think of plastic litter, you  probably do not think of cigarette butts.

Because if you do not smoke,  you might not even realize that the woven filters of  cigarette are made of plastic, not from fabric or cotton as  they might initially appear. This makes smoked cigarette butts one of  the worst plastic pollution offenders, beating out plastic bags and straws.

These fuzzy bits of cellulose acetate are  such a common form of litter, in fact, that human smoking habits may be butting into the lives of local  wildlife, especially in cities. In particular, many common songbirds have  fallen afowl of our nicotine addiction. [♪ INTRO] Urban songbirds have had to learn to make  do in some very unnatural conditions. Just consider the bright lights at night,  reflective glass, and constant noise prevalent in big cities and  even in suburban neighborhoods.

Birds are not perfect at dealing  with all these modern inconveniences, windows and lighting do cause a  lot of bird-building collisions. But birds can be resourceful creatures. And city litter has increased  the amount, and kinds, of material available for  them to build their nests.

Flexibility in nest construction  materials means a songbird parent-to-be can use twigs, grass and, yes,  even fluffy cigarette butt fibers to keep eggs and chicks cozy and safe. While parents provide insulating  warmth to their offspring, nests are constructed to  provide both warmth and safety. Away from predators.

Secure on a branch. And safe from other pests  like ticks and mites, too. Some species, like house sparrows and  house finches, add a special lining to their nests made of plants  specifically known to repel parasites.

Good job, mamas! Fewer parasites could mean healthier chicks,  so it's an evolutionarily smart move. And it turns out that city birds might  be deliberately choosing used cigarettes for the same pest-repelling reason.

Sounds gross - but it could  be a really beneficial move to make the most of these construction  materials readily available. Tobacco is well-known to  have anti-pest properties. This built-in chemical feature protects  the plant leaves from herbivorous bugs.

It’s not just plant pests it works against either. Nicotine, specifically, is  created by plants to deter pests and then it just happens  have an effect on our bodies. But also there may be other compounds in tobacco that are great at deterring biting  insects like ticks, as well.

Cedar, lemongrass, and citronella are  other examples of plants with established pest-repelling properties that you – and the  wild birds – might be more familiar with. Songbirds have long known what bug-spray companies have had to use science to figure out. Our city-dwelling birdies seem to  have adapted to using tobacco-laced cigarette butts for this same pesticide property.

In one study, researchers showed  that nests made of smoked butts hosted fewer parasites than  nests made of un-smoked butts. Smoked cigarette butts seem  best for warding off parasites because they’re full of these alkaloid compounds. Researchers think the birds  could be using smell to identify the ideal material to use in their nests.

It’s pretty smart, really,  for them to use an existing, adaptive behavior and adjust it to ensure  reproductive success in the big city. But this begs the question: is a mite-free  nest worth the cost of the cigarettes? To answer that, we need to know if tobacco  is as bad for birds as it is for people.

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Thanks to Arizona State University’s  College of Global Futures for supporting this video!  Now let’s get back to it. We do have some evidence that, yes,  exposure to chemicals from cigarettes is harmful to birds as well as humans. Cigarette filters, the plastic part  that eventually becomes the spent butt, concentrate the harmful  chemicals in smoked tobacco.

The existence of filters is  meant to help block, or filter, some of these chemicals  from getting to the smoker. And when you use that stuff to build a nest, parents and chicks get direct exposure  to those extra-concentrated chemicals. We know that the compounds in cigarette smoke cause damage to DNA in humans.

These compounds include nicotine,  heavy metals, even cyanide. In humans, this damage can eventually  lead to things like cancer in smokers. In chicks reared in nests  made from cigarette butts and exposed to those compounds through their skin, we see evidence of DNA and red blood cell damage.

But is it bad for the birds? As far as I know, they're  not smoking the cigarettes. Another unfortunate twist here is that  the damage isn’t just dealt to the chicks.

Parent songbirds, depending on how much time they spend on the nest, are also affected. In some species, like the house sparrow, males and females contribute  about equally to nest building. A 2017 study found no difference  in the amount of DNA damage between male and female parents.

Whereas in house finches,  the females spend more time building the nest and brooding chicks. The same study showed that they  incur a higher amount of DNA damage than their male partners. It’s like nature set up a  natural experiment for us.

What we don’t know is if that  damage impacts the mother’s success at staying healthy enough to  parent subsequent clutches. Small birds tend to have small life spans, so them developing cancer in old age isn’t likely. Cuz they're not going to…get to old age.

Instead, we’ll need more  studies to also monitor chicks as they grow up, allowing  scientists to study the effects over a generation or more  of city-dwelling songbirds. So, while it’s kinda cool that urban  animals have the behavioral flexibility to squeeze a benefit out of a cigarette  filter, it’s yet another reason to be mindful of how plastic pollution impacts  wildlife in some really unexpected ways. [♪ OUTRO]