No abstract

A colony (ca. 2,000 birds) of the Great Cormorant (Phalacrocorax carbo) is present in Shinobazunoike Pond, Tokyo, Japan. The cormorants feed in rivers and in Tokyo Bay. The major direction of their daily flight in winter is to the rivers, and in summer to the Bay (Fukuda 1985). The relative importance of the two feeding sites in terms of dietary contribution is unknown. Dietary studies conventionally depend on stomach contents, cast pellets, and field observations. Although useful to reveal birds' choice of prey species, such details cannot determine the relative dependence of cormorants on the two feeding areas. Stable carbon isotope analysis of bird tissues is a convenient way to measure such dependence, as it tells if the organic carbon in the tissues is terrestrial or marine in origin. Compared with conventional dietary studies, isotope analysis is less susceptible to day-by-day variation of prey species and is in principle free of numerical bias for easily identifiable foods. The two stable isotopes of carbon are x2C and •3C. In various biogeochemical reactions, they react at different rates, and the ratio •3C/•2C of various carbon reservoirs becomes different (Mizutani and Wada

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 °13C and °15N in naturally obtained foods like fish during 6 yr was °1%. Factory—manufactured pellets generally gave lower deviations (0.2% for °13C and °0.6% for °15N). Feathers showed an enrichment relative to food in both 13C and 15N, although its extent varied among different bird species. Isotope ratios for 13C and 15N 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.

No abstract

No abstract