Geographic Variation of Mercury in Breeding Tidal Marsh Sparrows of the Northeastern United States

Abstract Saltmarsh sparrows ( Ammospiza caudacuta ) and seaside sparrows ( A. maritima ) are species of conservation concern primarily due to global sea-level rise and habitat degradation. Environmental mercury (Hg) contamination may present additional threats to their reproductive success and survival. To assess site-specific total mercury (THg) exposure and identify environmental correlates of THg detection across a large portion of the breeding range where these species co-occur, we sampled blood from adult male saltmarsh and seaside sparrows at 27 marsh sites from Maine to Virginia, USA. The mean THg concentration (±1 SD) throughout the entire sampling range was 0.531 ± 0.287 µg/g wet weight (ww) for saltmarsh sparrows and 0.442 ± 0.316 µg/g ww for seaside sparrows. Individual THg concentrations ranged from 0.135–1.420 µg/g ww for saltmarsh sparrows and 0.153–1.530 µg/g ww for seaside sparrows. Model averaging from a suite of linear mixed models supported species-based differences in blood THg. On average, saltmarsh sparrows had 20.1% higher blood THg concentrations than seaside sparrows. We hypothesize that species-specific THg concentrations are influenced by differences in diet or foraging preferences between these species. We did not detect any correlations between sparrow THg concentrations and land cover characteristics surrounding sampled marshes or cumulative average annual precipitation.