Tracing nutrient allocation to reproduction with stable isotopes: A preliminary investigation using colonial waterbirds of Great Slave Lake

We investigated the use of stable-isotope analysis as a direct means of tra cing allocation of endogenous protein and lipid reserves to reproduction in five gulls (Larus canus, L. delawarensis, L. californicus, L. argentatus, L. philadelphia), four terns (Sterna caspia, S. hirundo, S. paradisaea, Chl idonias niger), and one jaeger (Stercorarius parasiticus) breeding on Great Slave Lake (GSL) in the Northwest Territories. Our approach was based on a ssumptions that (1) body tissues of birds just arriving at GSL from their a ssumed marine-associated wintering habitats would have stable-isotope ratio s more enriched than those of birds in equilibrium with the local GSL foodw eb, and (2) mobilization of these reserves to reproduction could be traced by the isotopic measurement of egg macronutrients. As predicted, the pector al muscle of six species of arriving birds was more enriched in C-13 ((x) o ver bar = -21.5 parts per thousand) and N-15 ((x) over bar = 12.7 parts per thousand) than was that of postbreeding birds (C-13, (x) over bar = -23.5 parts per thousand; N-15, (x) over bar = 9.9 parts per thousand) or hatchin g-year birds raised at GSL (C-13, (x) over bar = -24.3 parts per thousand; N-15, (x) over bar = 9.0 parts per thousand). Abdominal fat of arriving Her ring Gulls and Mew Gulls was more enriched in C-13 ((x) over bar = -19.7 pa rts per thousand) than the fat of other species ((x) over bar = -23.4 parts per thousand), indicating lipids of marine origin. We compared isotope val ues of the local GSL foodweb with dietary values predicted from isotope mea surements of egg macronutrients ii diets were entirely derived at GSL. Isot opic analysis of lipid-free egg yolk, yolk lipid, and shell carbonate sugge sted that for most species, little if any endogenous protein reserves were used for reproduction, with the possible exception of Caspian Terns, whose egg protein and egg lipid values, and Common Terns, whose egg protein value s, were more enriched in C-13 than those of the other species. Although end ogenous nutrient reserves likely were important to birds during migration a nd the initial settling period at GSL, local food supplies were adequate to provide nutrients for reproduction.