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Two Innate Immune Measures Increased With Urbanization in Adult Blackbirds, but Not Nestlings

Adult male Eurasian blackbird standing on grass in an urban garden
Research · Physiology

Adult Eurasian blackbirds (Turdus merula) breeding in more urbanized parts of the eastern Netherlands had higher values for two measures of innate immune function, while nestlings showed no comparable relationship with urbanization. The study compared birds from five sites during the 2021 and 2022 breeding seasons and measured urbanization as the proportion of buildings, roads and other impervious surfaces around each nest.

The researchers analysed three immune measures in blood plasma: haptoglobin, antimicrobial activity and haemagglutination. Haptoglobin is an acute-phase protein that rises during inflammatory responses. Haemagglutination measures the activity of naturally occurring antibodies by testing how strongly they cause red blood cells to clump together. These measures describe different parts of immune function and should not be read as a single scale from “weak” to “strong” immunity.

Two adult immune measures rose with urbanization

Haemagglutination increased with urbanization in adult blackbirds. Haptoglobin also increased most clearly in adults older than their second calendar year. Antimicrobial activity, by contrast, showed no relationship with urbanization.

Nestlings showed none of these urbanization relationships. They were sampled at about eight days old, when several parts of the immune system are still developing, and their haemagglutination response was not detectable. The contrast between adults and nestlings therefore may partly reflect age and immune development rather than a single effect of city life that applies throughout the birds’ lives.

Innate immune defenses of nestling and adult Eurasian blackbirds along an urbanization gradient
Haptoglobin concentration, antimicrobial activity and haemagglutination titre in nestling and adult Eurasian blackbirds along the urbanization gradient. Figure 2 from van Irsel et al. (2026), Ornithological Applications, CC BY 4.0.

Higher values do not simply mean healthier birds

The authors caution that higher haptoglobin and natural-antibody activity do not mean that urban blackbirds were simply healthier. Haptoglobin can rise during inflammation or infection, while greater natural-antibody activity may reflect altered immune investment or greater exposure to environmental challenges.

One proposed explanation is that urban diets may contain fewer dietary antioxidants, increasing reliance on immune-related compounds that can also help limit oxidative stress. The study did not measure diet, oxidative stress or pathogen pressure directly, so this remains a hypothesis rather than a demonstrated mechanism.

The adult sample was small, with immune assays available for 31 birds, and the study covered only five sites. The urbanization index also captured impervious surface rather than other aspects of city life such as food, pollution and parasite communities. The results therefore show age-dependent associations between urbanization and particular immune measures, not that urbanization generally strengthens or weakens the blackbird immune system.

Sources and image credit

  1. van Irsel, J., van der Jeugd, H. P., Huang, X. S., de Boer, W. F., Foppen, R. P. B., & Matson, K. D. (2026). Urbanization is associated with innate immune functions in adult, but not nestling Turdus merula (Eurasian Blackbird). Ornithological Applications, 128(1), 1–11.Primary peer-reviewed open-access article, published 30 January 2026. CC BY 4.0. Figure 2 is reused in the story under the article licence.

Image: Jules Verne Times Two — adult male Eurasian blackbird in Calouste Gulbenkian Garden, Lisbon, Portugal, photographed 20 March 2021; representative urban species image, not a bird from the Dutch study. Source: Wikimedia Commons. · CC BY-SA 4.0 · Source

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