Abandonment was the most frequent cause of nesting failure in Great Lakes Piping Plovers (Charadrius melodus) during 1993–2007, and observations of color-banded adults suggested that most abandonments involved disappearance of attendant adults rather than behavioral decisions by adults to desert their nests. The purpose of our study was to integrate nest-monitoring histories with mark—resighting analyses to determine whether nest abandonment indicated adult mortality or nest desertion. Nesting Piping Plovers had extremely high within-year resighting probabilities (10-day mean p = 0.908 ± 0.025 [SE] ), and cumulative probabilities of being detected approached 1 for individuals that were present on the breeding grounds for ≥20 days post-abandonment. These observations suggested that desertion would be readily identified unless Piping Plovers left the monitoring areas immediately after deserting their nests. None of the 31 that disappeared (as indicated by nest-monitoring histories) was ever observed again, and an among-year mark—resighting analysis suggested that all had died. From 2002 to 2007, annual mortality associated with disappearances averaged 5.7% of the marked population. Disappearances occurred primarily from 16 May to 19 June, were more common among females, involved individuals that were older than average, and were most frequently attributed to predation by Merlins (Falco columbarius). Our results reveal that most early-season nest abandonment in Piping Plovers was attributable to the death of attendant adults. We believe that this phenomenon may be widespread among other species of birds in which adults are vulnerable to mortality during nesting.