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Songbird reproductive success can decline from consuming mercury-contaminated aquatic insects, but assessments of hydrologic conditions influencing songbird mercury exposure are lacking. We monitored breast feather total mercury (THg) concentrations and reproductive success in the U.S. federally listed endangered Cape Sable Seaside Sparrow (CSSS: Ammospiza maritima mirabilis) over three breeding seasons in the Florida Everglades. We used model comparison to explore the influence of annual hydrologic variation on adult CSSS THg concentrations, and tested mercury effects on individual reproductive success (individuals’ mate status, apparent nest success, and total productivity) that were scaled to estimates on population productivity using a demographic model. We identified four hydrologic models that explained annual variation in adult THg concentrations, with the top model showing a negative association between THg concentrations and drought length of the previous breeding season and a positive association between THg concentrations and dry-season water recession rate (model adjusted R2 = 0.82). Adult male mating probability declined by 63% across the range of THg concentrations observed. We found no mercury effect on CSSS nest success or total productivity. However, demographic modeling suggested the reduced mating could produce a 60% decrease in population productivity compared to a scenario with no THg impact. Our results suggest that CSSS mercury exposure is influenced by local hydrologic conditions that can increase early breeding failure (lack of breeding initiation) and potentially limit population productivity. This study is the first to describe CSSS mercury exposure and its potential reproductive costs at the individual and population levels.