Species-specific and microbial variability in the antimicrobial efficacy of uropygial secretions in wild passerines

Uropygial secretions are thought to play an antimicrobial role in birds, but few studies have compared their effectiveness across multiple species and microorganisms using standardized methods. We evaluated the antimicrobial activity of uropygial secretions from 212 individuals of 11 passerine species in southwestern Spain, testing their inhibitory effects against Bacillus licheniformis, B. subtilis, Candida albicans, Escherichia coli, and Salmonella typhimurium by flow cytometry. Antimicrobial activity varied significantly among species and microorganisms: house sparrows (Passer domesticus) consistently exhibited stronger and broader responses, whereas activity against S. typhimurium was generally low. Phylogeny, migratory status, and social behavior did not explain variation, but uropygial gland size was positively associated with antimicrobial activity both globally and in some bird species. Body condition exhibited species-specific associations with antimicrobial activity, showing positive correlations in some bird species and negative correlations in others. Overall, our findings highlight the uneven distribution of antimicrobial defenses among passerines and underscore the importance of comparative, chemically informed, and methodologically standardized approaches to clarify the ecological and evolutionary significance of uropygial secretions.