Urban blue tit nests and cigarette butts: accidental litter or adaptive behaviour?

Birds are known to be able to actively shape their environment through their choices of habitat, behaviour or metabolic activities. Life-history theory suggests that parasites can generate selection pressures by affecting host health and, in natural habitats, Eurasian blue tits, Cyanistes caeruleus , are known for incorporating fresh herbal plants that contain volatile compounds into their nests to avoid parasites and their effects. In contrast, urban birds often come across resources not found in natural environments, including materials present in urban waste. There have been increasing reports that blue tits living in cities bring cigarette butts into their nests or build nests in cigarette ashtrays and butt bins. Tobacco, Nicotiana spp., is often cited as an efficient insect repellent and insecticide. In this study, we experimentally tested whether the inclusion of cigarette butts in nests (treatment 1), or replacing a natural nest with a sterilized, artificial, moss and cotton wool nest on the 5th and 10th day of the nestling period (treatment 2), affects components of nestling blue tit fitness: physiological condition, as indicated by blood levels of haemoglobin, glucose, haematocrit and body condition index (measured as the length of the wing of 13-day-old nestlings), in comparison with a control group. After the experimental procedures ended, we also collected all nest materials to extract, identify and count arthropod ectoparasites in the three treatment groups. Nestlings from broods supplemented with cigarette butts and nestlings in broods with artificial nests had significantly elevated haemoglobin and haematocrit, indicating improved physiological condition compared to the control group. The abundance of arthropods (ticks, mites, fleas and blowflies) in the nesting material varied depending on the experimental treatment, but, overall, ectoparasites were most numerous in natural nests, slightly less abundant in nests with cigarette butt addition and nearly absent in sterile nests. A reduction in blowfly larvae in nests with cigarette butts was observed compared with controls but this effect was only nearly significant. • Cigarette butts and sterile nests improved nestling blood parameters. • Sterile nests contained significantly fewer ectoparasites than natural nests. • Cigarette butts only marginally reduced blowfly and flea abundance.