We report the first known nest of the Pale-eyed Blackbird (Agelaius xanthophthalmus) and describe the behavior of breeding and nonbreeding pairs at Cocha Cashu, Manu National Park, Peru. Birds traveled as pairs within limited areas, whether or not they were breeding. During incubation, which is performed only by the female, the male typically remains near the nest and the birds are in regular vocal contact by means of frequent duets. Its monogamous social system is similar to that of other marsh-nesting species in its clade.