International study on the use of cigarette butts by urban birds highlighted in The New York Times

A new study published in the journal Animal Behaviour and recently featured in The New York Times explores a surprising behaviour observed in birds living in urban environments: the use of cigarette butts in nest construction. Researcher Ana Cláudia Norte, from MARE – University of Coimbra, was part of the international team led by the University of Łódź (Poland) that conducted this pioneering investigation.

The study seeks to understand how birds, particularly the blue tit (Cyanistes caeruleus), make use of materials found in urban waste to cope with ecological challenges, namely the presence of ectoparasites in their nests. In natural habitats, this species is known to incorporate aromatic plants rich in volatile compounds to reduce parasite loads. However, in urban settings, birds encounter alternative resources, including tobacco waste, which is often cited as an insect repellent.

An experimental study to assess physiological and parasitic effects

The team tested two experimental scenarios:

• the addition of cigarette butts to natural nests;
• the replacement of the natural nest with a sterilised artificial nest made of moss and cotton wool.

The researchers assessed indicators of nestling physiological condition, such as haemoglobin, haematocrit, glucose levels and body condition index, and quantified the presence of ectoparasites (ticks, mites, fleas and blowfly larvae).

The results showed that:

• nestlings in nests with cigarette butts and in artificial nests displayed better physiological condition, with higher haemoglobin and haematocrit levels;
• ectoparasites were most abundant in natural nests, less frequent in nests with cigarette butts and almost absent in sterilised nests;
• a reduction in blowfly larvae was also observed in nests with cigarette butts, although this effect was marginal.

How birds adapt to cities

The study reinforces the idea that birds are capable of actively modifying their environment, adjusting behaviours and nesting strategies to urban conditions. Although the use of cigarette butts may offer some protection against parasites, the authors emphasise that this behaviour results from exposure to human waste and should not be interpreted as ecologically or environmentally beneficial.

The participation of Ana Cláudia Norte highlights MARE’s contribution to international research that deepens our understanding of urban ecology and the ways in which wildlife responds to human-driven pressures.

Text by Zara Teixeira

Cover photo by Gabriel Almanzar na Unsplash