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The use of stable isotopes of bioelements to analyze diets of animals in nature requires precise knowledge of any changes in the isotope ratios that take place after assimilation. Therefore, the influence of diet on the distribution of carbon and nitrogen isotopes in feathers of 11 species of birds on a constant diet was studied. The standard deviation in ° 1 3 C and ° 1 5 N in naturally obtained foods like fish during 6 yr was °1%. Factory—manufactured pellets generally gave lower deviations (0.2% for ° 1 3 C and °0.6% for ° 1 5 N). Feathers showed an enrichment relative to food in both 1 3 C and 1 5 N, although its extent varied among different bird species. Isotope ratios for 1 3 C and 1 5 N from diet to feathers converged. The nature of the enrichment factors is discussed in relation to the convergence and temporal window size. Neither enrichment showed an age dependency among adult birds, and neither depended on the nitrogen content in the bird's diet. These results show that the stable isotope approach can be applied not only to these 11 species but to birds whose isotope enrichment factors are so far unknown.
Mizutani et al. (Sat,) studied this question.