Birds of prey and the city: variation in immunity and parasites exposure along an urbanization gradient

Pathogens are major ecological and evolutionary forces, and the immune system is central to coping with these challenges. Environmental variation, including urbanisation, can alter both pathogen exposure and immune function, making it important to understand how responses vary across habitats. We addressed the impact of urbanization on both immunity and parasite exposure using common kestrel Falco tinnunculus nestlings as our study species. This generalist raptor occurs in both urban and non‐urban areas, offering a model to investigate how urban environments affect early‐life immunity. Previous studies on kestrels reported baseline immune differences but did not test induced responses, limiting understanding of immune function. We challenged nestlings with phytohemagglutinin (PHA), collecting blood before and 24 hours after injection, and measured six immunological markers: haptoglobin, bacterial killing activity, lysozyme, hemagglutination, hemolysis, and IgY. We also quantified exposure to two types of ecto‐ and hemo‐parasites. PHA induced an inflammatory response, but this response was similar across habitats, indicating that the habitat did not influence the responsiveness to this particular antigen. Baseline innate immunity generally increased along the urbanisation gradient, whereas baseline adaptive immunity (IgY) rose within a season but in a similar way across habitats. Exposure to ectoparasites Carnus hemapterus was significantly higher in natural habitat compared to urban habitat, but did not explain individual variation in the response to the PHA challenge. Exposure to hemo‐parasites was negligible, as we found only two out of 108 nestlings positive. Our results suggest that urbanisation does not influence the immune response to an experimental immune challenge. Further studies are needed to determine whether these findings can be generalized to other antigens or live parasites, as well as to populations with differing histories of parasite exposure or habitat conditions.