We document the first recorded instance of a fish depredating a hatchling bird while in the nest. Some coastal marsh-nesting avian species, such as the Seaside Sparrow (Ammospiza maritima), experience tradeoffs between the threat of nest predation and nest flooding. Nesting too low may result in nest flooding during the twice-daily high tides and increasing nest height increases the risk of nest predation. However, not much is known about how nest height affects exposure to different predators. We observed a mummichog (Fundulus heteroclitus) enter a Seaside Sparrow nest during a nest flooding event and depredate a hatchling. This observation expands our understanding of the predator–prey relationships experienced by coastal marsh-nesting birds, and the potential detriments of intensifying sea level rise on their nesting success.