Burrowing Owls (Athene cunicularia hypugaea) are ground-dwelling owls of western North American grasslands and steppes. They hunt rodent prey and occupy burrows once inhabited by fossorial mammals and thus frequently harbor fleas. Fleas are noteworthy because they have the potential to vector plague through transmission of Yersinia pestis, the etiological agent. The predominant species of flea infesting burrowing owls is Pulex irritans (Family Pulicidae), the so-called human flea. The potential role of Pulex irritans in plague dynamics is the subject of ongoing investigation. Here, we studied the extent to which migratory Burrowing Owls transport fleas from wintering grounds, where plague is prevalent, to the breeding grounds. We examined ratios of stable isotopes of hydrogen, which vary geographically, in breeding-season collections of fleas, owl feathers and owl talons from Idaho and Oregon to compare isotopic signatures and decipher geographic origins of fleas. We tested the hypothesis that springtime fleas on adult owls, but not those from local nestlings, have wintering-ground origins by comparing stable isotope ratios in flea keratin to ratios in feathers and talons of adults and nestlings. Our study furthers understanding of Burrowing Owl host-parasite relationships and the ecology of plague in western landscapes.