Helminth parasites are key regulators of host health, population dynamics, and ecological interactions. Yet, the influence of urbanization on parasite prevalence in colonial waterbirds remains poorly understood. We investigated gastrointestinal parasites in Ardeola grayii across urban and semi-urban sites in North Kerala, India. A total of 150 fecal samples were collected and examined and we evaluated colony-level variables including site type (urban vs. semi-urban), nest abundance, and species composition (single vs. multi-species nesting).

Overall, the majority of samples (67.3%) were parasite-free, with a higher prevalence recorded in semi-urban sites (38.0%) compared to urban sites (24.1%). Logistic regression analysis revealed site category as a significant predictor (p = 0.01), indicating that birds in urban colonies had a lower probability of harboring parasites. In contrast, nest abundance (p = 0.10) and species composition (p = 0.60) showed no significant influence.

The lower prevalence of parasites in urban colonies likely reflects ecological shifts driven by urbanization. Contributing factors may include declines in intermediate host populations, altered foraging behavior of urban-adapted birds that rely on human-provided food, and potential pharmaceutical contamination in urban waterbodies that reduces parasite survival. Such findings align with broader ecological evidence that urbanization reduces biodiversity and host richness, thereby limiting opportunities for parasite transmission. This study underscores the importance of integrating parasite ecology into urban wildlife research. A deeper understanding of how urbanization shapes host–parasite interaction has significant implications for biodiversity conservation and for anticipating and managing zoonotic disease risks in rapidly developing landscapes.