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Connecting breeding and wintering habitats of migratory piscivorous birds: implications for tracking contaminants (Hg) using multiple stable isotopes.

Contaminant concentrations in migratory species are complicated by differential accumulation and elimination among geographically separated biomes. Double-crested cormorants ( Phalacrocorax auritus ) are used as monitors of Hg in North America; however, migration from northern breeding colonies to southern marine, freshwater, and aquaculture systems exposes individuals to spatiotemporal variations in contaminant uptake. We used stable isotopes (δ(34)S, δ(13)C, δ(15)N, δ(2)H) in primary feathers and a combined Bayesian assignment and isotopic threshold model to identify feather origins and the potential winter use of aquaculture (δ(34)S < 10‰, δ(13)C > -14‰), freshwater (δ(34)S < 10‰, δ(13)C < -20‰), and marine habitats (δ(34)S > 10‰). This approach allowed us to contrast body burden Hg derived from the breeding and wintering grounds, as well as from marine and freshwater habitats. We found feathers grown on Lake Winnipeg had greater Hg concentrations (mean = 4.26 ± 1.47 μg/g fresh weight; n = 20) than winter-grown feathers (3.19 ± 1.64 μg/g; n = 19), but Hg in winter-grown feathers was not related to any specific habitat. Isotopic assays of tissues of migratory birds allowed the source and degree of contaminant exposure to be identified throughout the annual cycle.