Changes in Stable Carbon and Nitrogen Isotope Ratios in Sooty and Short-Tailed Shearwaters during Their Northward Migration

Abstract

Migratory behaviors and associated food habits were investigated for two closely-related seabird species, Sooty (Puffinus griseus) and Short-tailed Shearwaters (P. tenuirostris), sampled in the western North Pacific from April to June in 1986, 1989, and 1990 using their carbon and nitrogen isotope ratios (muscle δ 13 C and δ 15 N). The shearwaters exhibited δ 13 C and δ 15 N values of −24.5 to −17.6‰, and 7.3 to 13.0‰, respectively. Shear-waters showed areal differences in their stable isotope ratios: those from the oceanic group in June had significantly higher δ 13 C and δ 15 N values than those from the coastal area of northern Japan in April and May. The isotope compositions of both species increased along with their migration from the Southern Hemisphere to the North Pacific. The low stable isotope ratios in the birds from the coastal area strongly demonstrated the preservation of isotope ratios of prey in the Southern Hemisphere before their northward migration. Significant differences in a δ 13 C-δ 15 N map were also observed between two species as well as within single species in time and space. In the coastal area of northern Japan, Sooty Shearwaters showed somewhat higher δ 13 C and δ 15 N values than Short-tailed Shearwaters. However, no interspecific difference of their δ 15 N value was found in the oceanic area of North Pacific, but the δ 13 C value of Sooty Shearwater was significantly higher than these of Short-tailed Shearwater. The interspecific difference in the coastal and oceanic areas reflects their changing dietary difference