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Mercury (Hg) concentrations in the blood and feathers of five species of migratory marsh birds, Nelson's sparrow (Ammodramus nelson subvirgatus), song sparrow (Melospiiza melodia), swamp sparrow (Melospiza geogiana), red-winged blackbird (Agelaius phoeniceus), and Virginia rail (Rallus limicola), breeding in marshes along the lower Penobscot River, Maine, far exceeded reference concentrations, exceeded concentrations associated with reproductive health, and are the highest Hg concentrations reported to date for several species. Blood Hg concentrations in adult Nelson's sparrows were greatest in 2007, at 6.6μg/gww (geometric mean) and in 2012, at 6.5μg/gww and greatest in red-winged blackbirds in 2012, 8.0μg/gww. Mercury in blood increased with residence time on the contaminated marshes at an estimated rate of 0.04 to 0.07μg/gww per day. Feather mercury concentrations in specific primary, secondary and tail feathers (P1, S2, R6) were strongly associated with exposure location at the time of feather formation. Geometric mean Hg concentrations in primary feathers (P1) reached 39.6μg/gfw in 2010 in Nelson's sparrows. The paper documents the dynamic nature of Hg concentrations in avian blood and feathers, an important consideration in contaminant study design, and the increased risk to marsh birds posed by Hg deposition from upstream sources., (Copyright © 2018 Elsevier B.V. All rights reserved.)