River Otter Predation of Nesting Seabirds Along the Coasts of North America

River otter (<i>Lontra canadensis</i>) predation of seabirds has been periodically documented throughout North America, but most knowledge about this subject exists as anecdotal observations in the memories of experienced seabird biologists. Using surveys, interviews, white papers, and published literature, we collected detailed records of seabird predation by river otters at 73 islands on the coasts of North America. From this dataset, we were able to draw conclusions about the seabird species experiencing river otter predation, the geographic distribution of predation records, and factors associated with river otter occurrence at colony islands. River otters preferentially select for small-bodied burrow-nesting species, with predation of ground-nesting species occurring mostly opportunistically. Predation of nesting seabirds occurs across the entire Pacific range of the river otter and throughout the northern Atlantic, but we did not locate a single record of this behavior on the Atlantic coast south of Maine; we hypothesize that this is due to geographic differences in nesting seabird assemblages. The presence of burrow-nesting seabirds and proximity to the mainland are both strong predictors of river otter occurrence at colony islands. River otters will swim up to eight kilometers across open ocean to access seabird colonies, but predation is significantly more common at nearshore islands (median distance of 1.7 km). Reports of seabird predation by river otters in published literature are rare, are heavily biased toward an ornithological audience, and have decreased over time as "Field Notes" style publications have decreased in popularity. Despite being more widespread than previously reported, predation by river otters appears to be sustainable for most colony nesting seabird populations.