Urbanization is associated with innate immune functions in adult, but not nestling <i>Turdus merula</i> (Eurasian Blackbird)

Abstract Increasing urbanization poses significant risks to organisms confronted with new and rapidly changing environments. Long-term exposure to urban environments may involve changes in the immune system and its interrelated processes. A properly functioning immune system is essential for health maintenance, including the prevention of infection, so identifying and understanding the links between urbanization and immunological changes can offer insights into wildlife health and disease emergence. Here, we investigated the relationship between three measures of innate immune defenses in nestling and adult Turdus merula (Eurasian Blackbird) and the level of urbanization. Two of these immune defenses were positively correlated with urbanization in adults, but no relationships were found between urbanization and immune defenses in nestlings. Additionally, no apparent trade-offs between body condition and immune defenses measures were observed. Our results suggest that urbanization does not negatively impact immune defenses in T. merula, but immune defenses may be upregulated in adult birds to cope with the environmental stressors related to urban habitats. Our study highlights the complexity of immune regulation in heterogeneous environments, such as urban environments, and emphasizes the need for more empirical research to disentangle how specific ecological pressures shape immune function across life stages.