STABLE-NITROGEN ISOTOPE ENRICHMENT IN AVIAN-TISSUES DUE TO FASTING AND NUTRITIONAL STRESS - IMPLICATIONS FOR ISOTOPIC ANALYSES OF DIET

Studies using measurements of deltaN-15 to delineate diet or trophic level in natural ecosystems are based on the premise that deltaN-15 values in consumer tissues can be reliably correlated with those in the diet. However, juvenile Japanese Quail (Coturnix japonica) fed a rationed diet designed to maintain, but not increase, body mass showed significantly enriched tissue deltaN-15 values over a control group fed the same diet ad libitum. We tested the hypothesis that fasting or nutritional stress can also cause elevated deltaN-15 values in tissues of wild birds by examining tissues of Arctic-nesting female Ross' Geese (Chen rossii) before and after their period of fasting during egg laying and incubation. Significant declines in body, pectoral muscle, liver and abdominal fat mass occurred from arrival through incubation. Post-incubating geese showed significantly higher pectoral muscle and liver deltaN-15 values compared to geese taken before clutch initiation but deltaC-13 values in these tissues were unchanged. We hypothesize a mechanism of tissue deltaN-15 enrichment due to reduced nutrient intake and discuss the implications of these results to ecosystem studies using stable-nitrogen isotope analysis.