Abstract The Hippoboscidae (Diptera) are a family of obligate blood‐feeding ectoparasites of birds (louse flies) and mammals (keds) that are known to vector pathogenic agents. Citizen scientists collected 4365 hippoboscids of 12 species, from 117 host species, in the UK, Ireland and the Isle of Man, as part of the ‘Mapping the UK's Flat Flies Project’. Of the 212 host—parasite interactions recorded, 70 were previously unreported in the region. Analyses of host characteristics showed evidence of niche separation by host size of the sympatric generalist species Ornithomya avicularia (L.) and Ornithomya fringillina (Curtis). Comparisons with data from a previous study, published in 1962, showed that all three generalist species in the genus Ornithomya increased their host associations during a period of climate and other anthropogenic changes: for example, the switch by some species of gulls (Laridae) to anthropogenic food sources has occurred over the same period that louse flies have started to parasitize them. These changes may have consequences for human and other animal health.