Use of stable-carbon isotope analysis to estimate marine and terrestrial protein content in gull diets

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Abstract
Stable-carbon isotope analyses of bone collagen of gulls (Larus spp.) were used to estimate the relative proportion of marine and terrestrial protein in the diets of gulls that are known to use both types of food sources. Mean .delta.13C values for Glaucous-winged Gulls (n = 23) collected at a Vancouver dump and for Western Gulls (n = 18) from Southeast Farallon Island were - 15.0 .+-. 1.3 and 15.1 .+-. 0.5.permill., respectively. No significant difference in .delta.13C values was found between adult male and female Western Gulls. Archaeological gull bones (n = 3) show a mean .delta.13C value of - 13.6 .+-. 1.0.permill. and support the assumed marine end point of - 13.0.permill.. The range of terrestrial protein in the diets of coastal gulls was 0-61%. The narrow distribution of .delta.13C values for Western Gulls from the Southeast Farallon Island colony suggests that gull colonies may be calibrated so that changes in dependence on terrestrial protein can be monitored.