When it comes to building a nest, only the best materials will do for a protective mother bird: she'll use all kinds of twigs, leaves and feathers to ensure her babies stay warm and safe. So why in the world are birds in Mexico City building their nests with cigarette butts?

In 2012, researchers tackled this question with science. A team at the National Autonomous University in Mexico examined the behaviour of house finches and house sparrows – both species that are common in densely populated urban environments where cigarettes are easy to come by – and found evidence that the birds use these nasty nest ingredients as pest repellant!

The birds weren't sticking whole cigarette butts into their nests. Instead, they would pull out cigarette butt fibres the same way they might grab a single twig or feather. As it turned out, the nests with the most fibres also tended to have the fewest parasites like ticks and mites, the sorts of little buggers that dine on the feathers and blood of developing chicks.

Momma birds were also picky about the cigarettes they'd use: they could tell the difference between smoked and unsmoked cigarette butts, and the research revealed a preference for the already-smoked variety. The scientists tested how unwanted visitors responded to these fibres and, as you might expect, creepy-crawly parasites actively avoided fibre-heavy nests, especially when the fibres came from smoked cigarettes.

Nicotine is a noxious substance, and smoked cigarette butts hold onto plenty of it, as well as other toxic compounds. It seems even blood-hungry parasites know better than to get too close to these poisons.

A soft-bodied tick beside its eggs. Ticks from the family Argasidae, like this one, are known to invade nests.

Could it be that these birds are intentionally using cigarette fibres to keep pests away? (It wouldn't be much more surprising than the owls that bring home pest-eating snakes.) Or are the finches simply grabbing the unusual nesting material for a different purpose, such as insulation, without even realising the helpful anti-parasite effects?

To dive deeper, the researchers went back for another study, published just last month in the Journal of Avian Biology.

This time, instead of seeing how ticks and mites respond to the presence of cigarette fibres, they investigated how finch parents responded to the presence of ticks. The team cleaned out several nests, and added live ticks to some, dead ticks to others, and left the rest alone, to see how momma birds would react.

The results? It looks like the birds know what they're doing. If the finches noticed ticks in their nests, they were much more likely to bring back cigarette fibres, especially if their nests housed live ticks as compared to dead ones.

It may seem like an incredible feat of intelligence for the finches to realise they can use cigarettes to keep pests away, but it actually isn't too much of a stretch from normal bird behaviour. Some birds are known to use toxic plants in their nests for this very same reason, for example – and plants are exactly where nicotine comes from.

Long before humans started using the stimulant for recreational purposes, the plant Nicotinia used it to dissuade animals from gnawing on its leaves – it's a natural bug repellant! And indeed, even humans have been known to use the substance to keep pests away from crops.

Just as with humans, however, the birds' cigarette habit may come at a cost. Part of the research on this finch-cigarette-parasite triangle has found that nests with more fibres also tended to have chicks with more genetic problems. It may be that the cigarette repellant is so beneficial that it outweighs the risks of deformed hatchlings, or perhaps urban living has led these resourceful birds to patterns of dangerous behaviour.



H/t: New Scientist 

Top header image: David A. Hofmann/Flickr