Urbanization’s Hidden Pleiotropic Effect on Pigeon Plumage Color and Blood Cholesterol

Some fries left on the ground, a half-eaten sandwich, or some breadcrumbs kindly given by someone on a park bench. Pigeons (Columba livia) in the city don’t shy away from eating human-derived foods. Although most pigeons are eager to eat our food scraps, this behaviour has been linked to a likely downside: the urban lifestyle of birds is associated with higher blood cholesterol levels, thought to be due to their human-food consumption¹. Although this link might seem straightforward, researchers recently found that there is more to this than meets the eye. New research found an additional risk indicator for high blood cholesterol in urban pigeons: the darkness of their plumage.

At a first glance this may seem like an unlikely link, but the group led by University of Chile researchers Javiera Arcila and Isaac Peña-Villalobos showed otherwise in their February 2025 article¹. In their research, they studied pigeons along a gradient of urbanization in and around the Chilean city of Santiago and found that degree of plumage melanism, or how dark a pigeon’s feathers are, correlated with the blood cholesterol of pigeons. Additionally, the researchers saw that pigeon plumage was on average darker in more urbanized environments. But why is this the case? How can feather color be related to cholesterol? And why do pigeons have a higher degree of melanism in the city?

The answer proposed by the researchers is about so-called pleiotropic effects, which are the effects that happen when a single gene affects two or more phenotypic characteristics. In this case, the two seemingly unrelated phenotypic characteristics are plumage color, and blood cholesterol. The underlying gene affecting both might be of the melanocortin system, say the researchers^1,2. The genes encoding this vertebrate system are highly conserved and they affect multiple processes in the body². Two processes affected by the melanocortin genes are melanogenesis and cholesterol homeostasis¹. Consequently, if environmental pressures favour a trait resulting from one of these processes, traits resulting from the other process will also be affected. So, in this case, that means darker pigeons will have higher average blood cholesterol levels. That leads us to the second question: why are pigeons darker in the city?

The environmental pressures of the city have resulted in some interesting differences between urban and rural pigeons, such as the darker plumage of pigeons in the city compared to pigeons in more rural suburban areas³. A possible reason for this phenomenon is that plumage color affects parasite prevalence, as researchers found that darker pigeons had less parasites than their lighter counterparts in an urban environment⁴.  The exact mechanism behind this is still unclear, but the prominent viewpoint in scientific debate is that the variability of pigeon colors is reflective of urban adaptation¹.

Researchers Javiera Arcila and Isaac Peña-Villalobos and their colleagues found a strong correlation between plumage melanism and blood cholesterol in pigeons. In their article, they showed that human-made landscapes, such as cities, can have broad and multifaceted effects on animal physiology, which consequently contributes to forming the evolutionary direction that a population moves in¹.

Want to know more? Read the paper here:

Arcila, J., Peña-Villalobos, I., Muñoz-Pacheco, C. B., Sanhueza-Cisterna, F., Galdames, D., Arancibia-Altamirano, D., Otárola, F. A., Landaeta, D., Basto, F. del, Aroca, C. C., Jiménez, T., Sabat, P., & Palma, V. (2025). Urbanization’s hidden influence: Linking landscape alterations and feather coloration with pigeon’s cholesterol levels. Environmental Research, 271, 121115. https://doi.org/10.1016/j.envres.2025.121115

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References

1. Arcila, J. et al. Urbanization’s hidden influence: Linking landscape alterations and feather coloration with pigeon’s cholesterol levels. Environ. Res. 271, 121115 (2025).
2. Gantz, I. & Fong, T. M. The melanocortin system. Am. J. Physiol.-Endocrinol. Metab. 284, E468–E474 (2003).
3. Csanády, A. & Duranková, S. Being Dark is Better: A Feral Pigeon Plumage Polymorphism as a Response to Urban Environments in Slovakia. Ekológia Bratisl. 40, 54–61 (2021).
4. Jacquin, L. et al. A potential role for parasites in the maintenance of color polymorphism in urban birds. Oecologia 173, 1089–1099 (2013).

Featured photo: Pxhere.com, CC0 Public Domain