Louse Flies on the Fly: Host Macroecology Shapes Interspecific Variation in Ectoparasite Prevalence Among Migrating Birds

ABSTRACT The prevalence of ectoparasites is strongly affected by the host environment, which shapes not only host exposure, but also parasite transmission and survival. Louse flies (Diptera: Hippoboscidae) are obligate blood‐feeding ectoparasites of birds and mammals, showing morphological adaptations to permanent life on the host body, such as dorsoventrally flattened bodies and strong claws. Here, we conducted a broad‐scale screening of louse fly prevalence across 157 avian host species captured during autumn migration through Central Europe (over 100,000 individuals screened). We also ran a phylogenetically informed comparative analysis to identify host traits shaping interspecific variation in louse fly prevalence. We identified seven louse fly species across the entire sample, including two dominant species ( Ornithomya avicularia and O. fringillina ). The median louse fly prevalence across all host species was low (median prevalence 2.2% infected host individuals; 67% infected host species). Phylogenetically‐informed Bayesian models revealed that the total louse fly prevalence was significantly associated with host species, body mass, precipitation within the host breeding range, host habitat type, trophic niche and migration distance. However, models run separately for the two most prevalent louse fly species ( O. avicularia and O. fringillina ) showed contrasting (species‐specific) effects of host traits. Our results revealed that interspecific variation in the prevalence of the obligate avian ectoparasites is shaped by a complex combination of host life‐history traits, habitat features and climatic conditions. The study provides novel insights into the ecological and environmental determinants of host–parasite associations, shedding light on how biodiversity patterns of parasites and their hosts are shaped across species and habitats. Because climate‐driven shifts in migration strategies and environmental conditions are likely to reshape host–parasite interactions, identifying host traits associated with susceptibility to ectoparasites is essential for predicting parasite risks and informing conservation.