Using stable-isotope analysis to identify endogenous and exogenous sources of nutrients in eggs of migratory birds: Applications to Great Lakes contaminants research

Stable-isotope analyses (carbon [C-13/C-12] and sulfur [S-34/S-32]) were performed on eggs of two migratory fish-eating birds, Caspian Terns (Sterna caspia) and Double-crested Cormorants (Phalacrocorax auritus) collected from Lake Ontario, Canada, to examine the extent to which nutrient reserves acquired on marine wintering grounds are transferred to eggs laid on freshwater breeding grounds. In order to establish isotopic patterns typical of eggs of birds using marine and freshwater C-3 biomes, eggs of Herring Gulls (Larus argentatus), a year-round resident on the Great Lakes, and those of Caspian Terns and Herring Gulls, breeding resepectively in the Gulf Coast of Texas and Atlantic Coast of Canada, were analyzed isotopically. Individual egg components showed distinct isotope values that were similar for both migratory and nonmigratory birds breeding in a freshwater biome and significantly lighter than those breeding in a marine biome. Hence, there appeared to be little evidence for significant nutrient transfer between the two biomes. The intermediate isotope values shown for egg components of Herring Gulls breeding on the Atlantic Coast suggest nutrient input from terrestrial as well as marine sources. Our results indicate the utility of stable-isotope analysis for tracing endogenous and exogenous contributions to reproduction in birds and further validate the use of migratory birds as indicators of breeding area contaminant levels and their effects on the Great Lakes.