Mercury exposure may be linked to several sources of variation related to habitat conditions and species ecology. In generalist birds, feeding habits may change quickly in response to environmental conditions, prey availability and individual requirements. Stable nitrogen isotope ratios (δ15N) were used as a marker of trophic level, and stable carbon isotope ratios (δ13C) as a marker of carbon sources (terrestrial vs. marine) in food webs involving waterbirds, to infer the effect on mercury exposure due to differences in feeding ecology and the relative dependence on aquatic environments. Four generalist species occupying three different habitats were examined during the breeding season. Habitats: Brackish water - saltwater (saltpans), brackish water - freshwater (ricefields and some saltpans) and terrestrial environment (steppes). Species: Avocet (Recurvirostra avosetta), Black-winged Stilt (Himantopus himantopus), Kentish Plover (Charadrius alexandrinus) and Cattle Egret (Bulbucus ibis). Chick feathers were collected at several locations between 2000 and 2003. Species used resources differently in the environment, and distinct pathways were involved in the mobilization of mercury into food webs. The positive relationship between feather δ15N values and mercury levels indicated mercury bioaccumulation and biomagnification. Inter-specific variation in feather δ13C values revealed a different relative dependence among species on terrestrial vs. aquatic prey. Intra-specific variation in feather δ13C values also indicated differential use of marine inputs within each species, and within saltpans for Avocet chicks. Feather mercury levels and δ13C values suggested that the relative use of marine-derived prey influences mercury levels in chicks.
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Vol. 32 • No. 2