Spatial and temporal differences in the diet of Great Lakes herring gulls (Larus argentatus): evidence from stable isotope analysis

Stable nitrogen (delta(15)N) and carbon (delta(13)C) isotope values were me asured in lipid-free homogenates of herring gull (Larus argentatus) eggs co llected from the Laurentian Great Lakes during 1974-1995. delta(15)N values provided an indication of adult gull trophic position during egg formation . After adjustment of egg delta(15)N values for interlake differences in ba seline food web delta(15)N signatures, significant differences in gull trop hic position were found among colonies. These results were attributed to di ffering proportions of fish in the diets of gulls from the various colonies . Aquatic foods available to gulls had greater delta(15)N values than terre strial foods. delta(13)C measurements provided some support for our hypothe sis that terrestrial foods, particularly garbage, were more enriched in C-1 3 than aquatic foods. As the proportion of fish in the diet decreased, the fraction consisting of terrestrial food increased, resulting in an increase in delta(13)C values and a decline in delta(15)N values. Stable isotope va lues for gull eggs from Lake Erie changed significantly during the study pe riod and may have reflected a decline in fish availability.