Increased mortalities of adult Eastern Bluebirds, Sialia sialis, breeding in artificial nesting boxes were recorded in New York State, US. A total of 46 dead bluebirds were reported from 23 sites between early April and mid-August 2017. The maximum distance between sites was over 600 km. A total of 27 carcasses were available for postmortem examination. The most common cause of death was necrotizing enteritis, found in 56% (9/16) of birds that could be examined histopathologically. Lesions consisted of foci of hypereosinophilic debris and inflammation rich in Gram-negative bacteria. Aerobic and anaerobic culture of intestines from 4/8 birds with necrotizing enteritis yielded no growth. Plagiorhynchus cylindraceus acanthocephalids were often (6/9, 67%) but not invariably present in affected birds. Occasional incidental lesions included foreign-body microgranulomas in the wall of the ventriculus and intravascular nematodiasis at the base of the heart. The cause of sporadic outbreaks of necrotizing enteritis in breeding bluebirds remains undetermined and warrants further investigation.