Nicotine is one of nature's bug zappers. Seriously. Lots of plants have evolved to produce bug-repelling chemicals as part of their defense mechanisms and tobacco happens to be one of those plants.
So when city-dwelling birds use the fluffy, nicotine-soaked material from discarded cigarette butts to build their nests it might not be the unmitigated ecological disaster that most of us imagine when we hear that "birds are building nests out of discarded cigarette butts". Researchers at Mexico's Autonomous University of Tlaxcala think the nicotine in the cigarettes might help keep chicks healthy — essentially serving as an urban substitute for the parasite-repelling plants the birds would have used in the wild.
At Culturing Science, Hannah Waters explains the idea…
But birds are actually quite fond of the chemicals found in some smelly plants, otherwise known as aromatics, from which "essential oils" are derived. Aromatic plants produce these chemicals to defend themselves against insects and other animals that would take them for food—but birds have their own use for them. Some nest-building species, including starlings and blue tits, regularly replenish their nests with fresh aromatics, and scientists hypothesize that the birds use these chemicals as parenting tools.
How would plant-derived chemicals help birds raise their chicks? It's possible that the chemicals boost the immune systems or development of the chicks so that they survive better after they leave the nest; this is known as the "drug hypothesis." Alternatively, the "nest protection" hypothesis suggests that the plant chemicals act as insecticides, driving parasites and other harmful insects from the nest.
Nicotine is an insecticide, although we don't often think of it that way. Tobacco plants generate nicotine because it defends against herbivorous beetles that would otherwise devour the plants–which means a smoker's buzz is caused by a plant's chemical defense mechanism. Some remnants of that insecticide remains in cigarette butts left in city streets, which are then transported into bird nests.
A house sparrow stands near a cigarette butt in Mexico City. Photo Credit: © Víctor Argaez