Stable-Isotope Ratios of Carbon and Nitrogen in Feathers Indicate Seasonal Dietary Shifts in Northern Fulmars

Ratios of the stable isotopes of nitrogen (•SN/•4N) and carbon (•3C/•2C) have been used to elucidate structure and interaction within food webs.There is a stepwise enrichment of •SN at each trophic level (Minagawa and Wada 1984, Schoeninger and DeNiro 1984, Hobson and Welch 1992, Hobson 1993).Carbonisotope ratios give weak trophic information, but indicate relative contributions from differing sources (e.g.marine vs. terrestrial; biota from these two systems having distinct b•3C signatures based on differences in b•C of carbon incorporated at photosynthesis) to the diet of particular species (Fry et al. 1983, Dunton and Schell 1987, Hobson 1987,1990, Mizutani et aL 1990, Hobson and Sealy 1991).When combined, stable-isotope ratios of nitrogen and carbon can be used to discriminate between species on the basis of dietary input to a greater extent than that afforded by one ratio alone (Hobson 1991, 1993).However, most such studies to date have utilized internal tissues (especially collagen extracted from bone or muscle), an approach that requires the killing of individuals in investigations of contemporary systems (Hobson 1990, 1991, Rau et al. 1992, Hobson 1993, Hobson et al. 1994).In contrast, Hobson and Clark (1993) provided isotopic information based on the analysis of avian blood, while Schaffner and Swart (1991) analyzed stable-isotope ratios of oxygen and carbon from eggshells to assess the diet of laying female seabirds.In studies of birds, feathers offer a further nondestructive alternative to the use of internal tissues, yet potentially should provide information relating to the diet and trophic position of an individual.However, few studies have made use of feathers in this way.A prerequisite for using feathers is an assessment of the fractionation of isotopes between the bird's diet and feathers.Such information is required to enable accurate interpretation of feather-isotope signatures when dietary preferences or trophic status are to be inferred, the implicit assumption being that isotopic signatures of diets can be related to those of consumers in a predictable manner.Feathers also of-' •t,• is diet-feather fractionation factor.