Stable nitrogen (δ15N) and carbon (δ13C) isotope values were measured in lipid-free homogenates of herring gull (Larus argentatus) eggs collected from the Laurentian Great Lakes during 1974-1995. δ15N values provided an indication of adult gull trophic position during egg formation. After adjustment of egg δ15N values for interlake differences in baseline food web δ15N signatures, significant differences in gull trophic position were found among colonies. These results were attributed to differing proportions of fish in the diets of gulls from the various colonies. Aquatic foods available to gulls had greater δ15N values than terrestrial foods. δ13C measurements provided some support for our hypothesis that terrestrial foods, particularly garbage, were more enriched in 13C than aquatic foods. As the proportion of fish in the diet decreased, the fraction consisting of terrestrial food increased, resulting in an increase in δ13C values and a decline in δ15N values. Stable isotope values for gull eggs from Lake Erie changed significantly during the study period and may have reflected a decline in fish availability.

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Journal Canadian Journal of Fisheries and Aquatic Sciences
Hebert, C.E, Shutt, J.L. (J. Laird), Hobson, K.A. (Keith A.), & Weseloh, D.V.C. (D. V. Chip). (1999). Spatial and temporal differences in the diet of Great Lakes herring gulls (Larus argentatus): Evidence from stable isotope analysis. Canadian Journal of Fisheries and Aquatic Sciences, 56(2), 323–338.