When to commence breeding is a fundamental decision made by individuals that inhabit seasonal environments. Although photoperiod determines the timing of breeding in most temperate zones, other abiotic conditions can also play a significant role by influencing food availability and, consequently, reproductive performance throughout a breeding cycle. This study used the multibrooded endangered Cape Sable Seaside Sparrow (Ammodramus maritimus mirabilis) to test whether water conditions (rainfall and groundwater levels) influenced breeding variables in a subtropical environment, the Florida Everglades. Timing of breeding was related to rainfall preceding the breeding season, with females initiating nesting up to 1 month earlier in years with greater rainfall. Clutch size averaged 3.4 eggs, and females showed an increase in clutch size as the breeding season progressed and in response to higher groundwater levels during the laying period. This effect was more apparent for first nesting attempts, with drier conditions limiting clutch size. Although wetter conditions favored earlier breeding and larger clutch sizes, annual nest survival (range: 12–36%) was negatively associated with high average rainfall late in the breeding season. Clutch-size variation and high nest survival in Cape Sable Seaside Sparrows' first nesting attempts suggests that food-mediated processes affect their reproductive decisions early in the breeding season, whereas predator-mediated processes drove overall reproductive output, possibly through increased activity of major nest predators during wetter conditions.