Inland vagrancy and overland dispersal by seabirds are rare phenomena for which the causes and consequences are poorly understood. Studying inland occurrences of seabirds is important because continental land masses form barriers to dispersal of genes and parasites; thus, the permeability of land barriers influences the extent to which seabird populations evolve on separate trajectories and co-evolve with distinct parasite faunas. The boobies (Sula spp.) are a clade of saltwater-obligate species occurring in tropical and subtropical oceans. Booby species tend to exhibit population genetic structure among oceanic regions. Although they rarely wander inland, the Brown Booby (Sula leucogaster) has occurred numerous times as a vagrant in interior North America. Previous inland records from far western North America originated from Pacific populations (subspecies brewsteri), whereas those from eastern and midwestern North America originated from Caribbean populations (subspecies leucogaster). In this paper, we describe 2 new specimens of S. leucogaster, first state records for New Mexico and Colorado, salvaged from localities nearly equidistant from Pacific and Caribbean source populations. We used mitochondrial DNA sequences to conclusively demonstrate that both specimens originated in the Caribbean. We next examined the helminth parasite fauna of the 2 S. leucogaster specimens and an inland-vagrant Blue-footed Booby (S. nebouxii) from New Mexico. The guts of these boobies contained a suite of helminths, including multiple freshwater-obligate taxa. Based on our findings, we suggest that susceptibility to freshwater parasites may cause overland dispersal to fail for boobies, or other seabirds. Thus, freshwater parasites may contribute to explaining the relative rarity of transcontinental dispersal, maintenance of strong inter-ocean population genetic structure, phylogenetic conservatism of specialization on salt-water habitats, and the strictly pelagic distributions of ocean-dwelling species from several avian families.

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