Zoonotic bacterial dynamics in two sympatric urban gull species within anthropized environments

Zoonotic infections remain a major global health concern, with Salmonella spp. and Campylobacter spp. among the most frequently reported pathogens in the European Union. Understanding how wildlife behaviour influences the dispersal and dynamics of these bacteria is crucial for assessing transmission risks in anthropized environments. Here, we examined the role of two sympatric gulls with contrasting ecologies, Audouin's gull (Ichthyaetus audouinii) and the yellow-legged gull (Larus michahellis), as biovectors of zoonotic bacteria. We combined GPS tracking of adults (2023-2024) with feather stable isotope data and bacterial screening of chicks (2022-2024) to examine interspecific and interannual variation in the prevalence of Campylobacter spp. and Salmonella spp., and to explore potential exposure and transmission pathways. Campylobacter spp. was detected only in Audouin's gulls, whereas Salmonella spp. occurred at low prevalence in both species. In part, these differences may reflect distinct exposure patterns linked to contrasting mobility, habitat use and diet. Audouin's gulls frequently used marine, coastal and industrial areas, and occupied a narrower, more marine-oriented niche, thereby increasing contact with Campylobacter spp. In contrast, yellow-legged gulls concentrated in urban areas and landfills and exhibited a broader, more terrestrial niche, which likely favours exposure to Salmonella spp. Overall, species-specific spatial and trophic ecology strongly shaped zoonotic dynamics. Integrating microbiology, GPS tracking, and isotopic markers offers an effective framework for identifying transmission routes and pathogen risks in anthropized landscapes supporting One Health-based management strategies.