Disentangling urbanisation, climate effects and their interaction on ornamental colourations
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Abstract
Urbanisation and climate change can both affect phenotype expression across taxa. Most evidence collected so far has focused on exploring these two phenomena on isolation. Currently, the combined effects of climate change and urbanisation remain underexplored, despite being among the greatest challenges faced by biodiversity. Here, we use a decade-long, individual-based study of urban and forest great tits ( Parus major ) to analyse urbanisation, climate and their interactive effects on yellow breast colouration, a carotenoid-based trait. We find that urban birds exhibit duller colourations than forest counterparts, being 10-20% less chromatic, with first-year birds and males being more negatively impacted by cities. Additionally, birds in the city are more sexually dichromatic than in the forest. Over the decade, colouration differences between habitats remain stable, following a similar quadratic temporal pattern. Finally, while climate has a weak effect on colouration, urban birds appear more sensitive to its influence than forest birds. Our results indicate urbanisation has a stronger impact than last decade climatic variation on great tit colouration, though both factors may interact. The heightened sensitivity of first-year birds and increased sexual dichromatism in cities may alter the strength of natural and sexual selection on this trait in urban environments.
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Cities pose many challenges for the organisms that inhabit them, including pollution and the “urban heat island” effect (Grimm et al., 2008). These factors have the most visible impact on species diversity, but their effects can also be observed at the level of individual physiological strategies in urban populations. One of the examples studied is the change in colouration of city birds. Specifically, the urban populations of songbirds are generally paler than forest populations, an observation named “urban dullness” (Salmón et al. 2023). The most frequently studied trait in this regard is colour intensity measured as the quantity of carotenoids deposited in feathers, while the remarkably less frequently investigated are traits associated with structural colouration (Janas et al. 2024).
Sandmeyer et al. (2025) compared the three …
Cities pose many challenges for the organisms that inhabit them, including pollution and the “urban heat island” effect (Grimm et al., 2008). These factors have the most visible impact on species diversity, but their effects can also be observed at the level of individual physiological strategies in urban populations. One of the examples studied is the change in colouration of city birds. Specifically, the urban populations of songbirds are generally paler than forest populations, an observation named “urban dullness” (Salmón et al. 2023). The most frequently studied trait in this regard is colour intensity measured as the quantity of carotenoids deposited in feathers, while the remarkably less frequently investigated are traits associated with structural colouration (Janas et al. 2024).
Sandmeyer et al. (2025) compared the three measures of carotenoid-based colouration of feathers, yellow chroma and two structural traits (mean brightness and UV chroma) in forest and urban habitat of great tit. The authors compared their results with temperature and precipitation trends for a 10-year period. In addition, the study considered the effects of the birds' sex and age.
The authors confirmed the existence of “urban dullness” in the studied populations, with city birds being more sexually dichromatic than forest birds. Contrary to expectations, the authors found no evidence that temperature or precipitation affected the birds colouration. However, the reason may be that all three measured traits showed non-linear trends over the years (despite the linear increase in temperature), which were consistent for both urban and forest habitats. This result is particularly interesting because it suggests the existence of an important environmental factor, other than temperature and precipitation, that plays a role in the colouration of bird feathers.
The authors correctly point out that, for proper evolutionary inference, it is important to examine whether urban dullness is a non-adaptive phenomenon caused by factors such as e.g. lower-quality diet (insects) in cities, or an adaptation to the achromatic urban environment.
References
Grimm N. B. et al. 2008. Global change and the ecology of cities. Science 319, 756-760, https://www.science.org/doi/10.1126/science.1150195
Janas, K., Gudowska, A., Drobniak, Sz. M. 2024. Avian colouration in a polluted word: a meta-analysis. Biological Reviews 99, 1261-1271, https://onlinelibrary.wiley.com/doi/10.1111/brv.13067
Salmón, P. et al. 2023. Urbanisation impacts plumage colouration in a songbird across Europe: evidence from a correlational, experimental and meta-analytical approach. Journal of Animal Ecology 92, 1924-1936, https://besjournals.onlinelibrary.wiley.com/doi/10.1111/1365-2656.13982
Lisa Sandmeyer, David Lopez-Idiaquez, Amelie Fargevieille, Pablo Giovannini, Samuel Perret, Maria Del Rey, Anne Charmantier, Claire Doutrelant, Arnaud Gregoire (2025) Disentangling urbanisation, climate effects and their interaction on ornamental colourations. bioRxiv, ver.6 peer-reviewed and recommended by PCI Ecology https://doi.org/10.1101/2025.02.19.639098
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