Mortality events have been reported in common eiders (Somateria mollissima) from Cape Cod, Massachusetts dating as far back as the mid-twentieth century. In 2006, a novel enveloped RNA virus was isolated from the tissues of birds from some mortality events that were investigated. Full-length genome sequencing demonstrated that this virus was an orthomyxovirus in the genus Quaranjavirus, and the name Wellfleet Bay virus (WFBv) has been proposed. Between 2009 and 2014, diagnosticians with the Southeastern Cooperative Wildlife Disease Study in Athens, GA, USA, examined seventy-nine common eider carcasses from mortality events in Massachusetts. The gross and histologic lesions seen in naturally infected common eiders will be discussed. Diagnostic testing, including a combination of immunohistochemistry (IHC), virus isolation (VI), serology, and reverse transcription nested polymerase chain reaction (RT-nPCR), confirmed 24 birds as positive for WFBv, 26 as suspect, 25 as negative, and 4 as previously exposed to WFBv without evidence of active infection. Gross findings associated with detection of WFBv were multifocal to coalescing, flat, tan foci disseminated on the surface of the liver and splenomegaly. Histologically, multifocal to coalescing foci of coagulative necrosis throughout the liver were associated with detection of WFBv. Liver, esophagus, and spleen were most consistently positive by VI. Molecular testing using RT-nPCR was more sensitive than other ancillary tests. Non-specific staining with IHC made interpretation of results challenging, but this test can be useful with concurrent ancillary testing and the presence of characteristic lesions. Further investigation of diagnostic techniques and characteristic lesions, will facilitate understanding of morbidity and mortality factors affecting common eider, as well as any potential population-level impacts of this recently discovered virus.