Influence of Lipid and Uric Acid on δ13C and δ15N Values of Avian Blood: Implications for Trophic Studies

The use of nitrogen (15N/14N) and carbon (13C/12C) stable isotopes to infer trophic relationships in food webs has become a common and widely accepted technique (see Michener and Schell 1994). Stable-isotope ratios in the tissues of consumers reflect those in their diets in a predictable fashion, being isotopically enriched in 15N (up to 4‰) and 13C (usually ca. 1‰; DeNiro and Epstein 1981, Peterson and Fry 1987). Thus, unlike conventional dietary studies, stable-isotope analyses reflect assimilated (vs. merely ingested) foodstuffs. In addition, depending on the tissue chosen, dietary information spanning different temporal scales can be obtained (Hobson and Clark 1992a). Typical proteins used in such analyses are bone collagen and muscle (e.g. Hobson 1987, 1990; Hobson et al. 1994), which usually require the sacrifice of live animals or the opportunistic sampling of carcasses. Isotopic analyses can also be performed on tissues that can be sampled nondestructively, such as blood, feathers, and hair (Hobson and Clark 1993, Thompson and Furness 1995, Bearhop et al. 1999). Nondestructive sampling is desirable when dealing with rare species, when individuals are part of ongoing studies, and when investigating variation within individuals over time. However, there are consistent differences in isotopic signatures among tissue types (e.g. Tieszen et al. 1983). Some of this variation can be linked to differences in tissue metabolic rates, but some reflects differences among tissues in biochemical composition (e.g. Hobson and Clark 1992a). Lipid presents a particular problem. The lipid component of a given tissue type can be quite variable among individuals and generally is depleted in 13C compared with whole tissues (e.g. Tieszen et al. 1983). For these reasons, it is common practice to remove lipids from tissue samples where they may be present (Hobson 1987, 1990; Hobson and Clark 1992b; Thompson and Furness 1995).