ABSTRACT: Tidal salt marshes face numerous pressures that threaten their ecological integrity, and many salt marsh vertebrates have exhibited declines concomitant with the degradation of salt marsh habitats. We studied nest survival of MacGillivray’s seaside sparrow Ammospiza maritima macgillivraii and Worthington’s marsh wren Cistothorus palustris griseus in smooth cordgrass Spartina alterniflora and black needlerush Juncus roemerianus dominated salt marshes in northeast Florida, USA, during 2015-2017 to understand why both species have undergone range contractions. We evaluated the effects of tides, habitat features, and weather on nest survival. The probability of a nest surviving from laying to fledging young was low for both species (seaside sparrow: 0.03, 95% CI: 0.00, 0.10; marsh wren: 0.10, 95% CI: 0.04, 0.18), suggesting that nest survival may limit population growth. The most substantial threat to marsh wren and seaside sparrow nest survival was from greater than normal high tides. However, there was limited evidence of direct nest loss due to flooding. An analysis of only fledged and depredated nests suggests that higher tides influenced nest survival by increasing nest predation, substantially so for marsh wrens. Other factors negatively associated with marsh wren nest survival, albeit less strongly, included precipitation and the amount of black needlerush at and within 50 m of the nest. The effect of tide height on nest survival suggests that salt marsh birds in northeast Florida are vulnerable to continued sea-level rise, which is exacerbated by upland development that will hinder marsh migration. Monitoring and protecting northeast Florida’s high-elevation, cordgrass-dominated marshes should be a priority.