We tested the hypothesis that malarial parasites (Plasmodium and Haemoproteus) of black-throated blue warblers (Dendroica caerulescens) provide sufficient geographical signal to track population movements between the warbler's breeding and wintering habitats in North America. Our results from 1083 warblers sampled across the species' breeding range indicate that parasite lineages are geographically widespread and do not provide site-specific information. The wide distribution of malarial parasites probably reflects postnatal dispersal of their hosts as well as mixing of breeding populations on the wintering range. When compared to geographically structured parasites of sedentary Caribbean songbirds, patterns of malarial infections in black-throated blue warblers suggest that host-malaria dynamics of migratory and sedentary bird populations may be subject to contrasting selection pressures.