We measured the fractionation of stable nitrogen (δ¹⁵N) and carbon (δ¹³C) isotopes in the breast and primary feathers of 11 Common Murres (Uria aalge) maintained on a diet of capelin (Mallotus villosus). Diet–feather δ¹⁵N fractionation from delipidated capelin muscle to murre feathers was 3.6‰ ± 0.2‰ in breast feathers and 3.7‰ ± 0.2‰ in primary feathers. Fractionation of δ¹³C was 2.5‰ ± 0.2‰ in breast feathers and 1.9‰ ± 0.3‰ in primary feathers. Prey–feather fractionation (for delipidated, muscle-only prey samples) for nine other species of seabirds ranged from 3.0‰ to 4.6‰ for δ¹⁵N and 0.1‰ to 2.5‰ for δ¹³C. Studies that did not remove lipids from prey samples showed higher δ¹⁵N and δ¹³C fractionation, and those that used whole prey items rather than muscle tissue alone showed higher δ¹⁵N fractionation. We suggest that: (1) prey samples be delipidated to facilitate interpretation of δ¹³C fractionation, (2) high interstudy and interspecific variation in δ¹³C makes species-specific studies essential, and (3) use of muscle tissue rather than whole bodies of fish will minimize unexplained variation in δ¹⁵N fractionation.