Fig. 1 American Robin nest, made of twigs and mud (Source: Mark on Flickr https://www.flickr.com/photos/eggrole/7213149990/, photo rotated by Kit Straley).
Birds can build their nests out of many different things – from the classic combination of twigs and mud to more creative compositions of grasses and even fresh plants. Birds that nest in urban places like cities or the surrounding suburbs have access to entirely different building materials for their nests: our trash.
Fig. 2 A bird nest with trash built in (Source: k.steudel on Flickr https://www.flickr.com/photos/snaks/3453764253/).
But why build a trash nest when twigs, grasses, and mud are still available choices? A study  by Suárez-Rodríguez and colleagues from the National Autonomous University of Mexico indicated that some trash could have benefits for chicks that grow up in the nest. Specifically, they investigated the use of cigarettes in bird nests and how the presence of cigarettes decreased parasites.
Parasites are creatures that live on other plants, fungi, or animals and are harmful to them. In birds, ectoparasites (parasites that live on the outside of the body) can have severe negative health effects on their hosts. Ticks, lice, and mites are just some of the ectoparasites that birds have to live with, even in the nest. These unwanted guests latch on to their hosts and live off of their blood, sometimes even killing their host .
Fig 3 Field Sparrow with 2 ticks on its face (Source: Alan Schmierer on Flickr).
Some birds combat this parasitic invasion by including fresh aromatic plants (plants that emit strong smells, like mint) in their nests. The plants contain chemical compounds that can deter or kill the parasites . Yes, that’s right, birds know how to fumigate.
In urban areas, aromatic plants may be few and far between but cigarette butts are abundant. Two bird species, the house sparrow and the house finch, have been observed using cigarettes in their nests . Suárez-Rodríguez’s team wanted to know if house sparrows and house finches in Mexico City were using our discarded butts to protect their chicks from harmful mites.
Fig. 4 A cigarette pack discarded in the woods (Source: StockyPics on Flickr).
The scientists used traps to capture ectoparasites around bird nests. Traps were baited with fibers from smoked and unsmoked Marlboro cigarettes, as smoked butts would release more anti-parasite chemical compounds like nicotine. At the end of the nesting period, they collected nests and parasites to see if nests with more cigarette fibers had more mites.
Fig. 5 Cigarette fibers and filters built into a swallow nest (Source: Bear Paw Battlefield on Flickr https://www.flickr.com/photos/bearpaw/5910472344/)
And the cigarettes worked! Nests with more cigarette fibers had fewer ectoparasites, and traps with smoked butts had fewer ectoparasites than traps with unsmoked butts. Surprisingly, over 80% of both bird species’ nests had cigarette fibers in them, and some nests had over 30 cigarette butts in them !
Am I recommending that we go around disposing of our cigarettes in bird nests? Of course not. Twiggy nests are basically piles of kindling, and that would be horrific. I don’t recommend littering cigarette butts either. The results of the study do show surprising beneficial uses of human litter in a world where our trash is almost exclusively bad for wildlife.
 Suárez-Rodríguez, M., I. López-Rull, and C. Macías Garcia. 2013. Incorporation of cigarette butts into nests reduces nest ectoparasite load in urban birds: new ingredients for an old recipe? Biology Letters 9(1):20120931. http://dx.doi.org/10.1098/rsbl.2012.0931
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