Abstract
Urban areas, i.e. dense housing and reduced green spaces, can significantly impact avian health, through altering land use and increasing biotic and abiotic stress. This study assessed the influence of urbanization on Haemosporidian infections, vectors, immune response, and body condition in Parus major nestlings, across four classes of urbanization along an urban-to-rural gradient in Vienna, Austria. Contrary to our expectations, vector abundance remained largely consistent across the gradient, while urban environmental factors, particularly impervious surfaces, significantly influenced Haemosporidian intensity. We found that nestlings in more urbanized areas exhibited higher parasite intensities and altered immune responses, as evidenced by variations in the heterophil to lymphocyte ratio and leucocyte counts. Moreover, nestlings from smaller clutches faced higher infection risks, suggesting increased probabilities to precept infections. Urbanization negatively impacted nestling body condition, with a decrease in fat deposits from forested to highly urbanized areas. Our findings highlight the complex interplay between urbanization, vector-borne parasite transmission, and host immune response, emphasizing the need for comprehensive urban planning. Understanding how urbanization affects bird immunity and parasite infections is critical for adapting urban landscapes for wildlife health and ecosystem integrity.