Ardea
Official journal of the Netherlands Ornithologists' Union

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Hobson K.A. (2006) Using stable isotopes to quantitatively track endogenous and exogenous nutrient allocations to eggs of birds that travel to breed. ARDEA 94 (3): 359-369
Conventional approaches to measuring endogenous nutrient allocations to reproduction in birds have relied on the sampling of several individuals within populations in order to contrast mass gains of the clutch with mass loss of the laying female’s tissues. The reasoning has been that mass loss of female endogenous tissues during laying can be attributed to ‘capital’ investment into reproduction. Apart from the destructive nature of this work, problems associated with this approach involve uncertain conversion efficiencies between endogenous reserves and eggs and the general loss of information on individual strategies. As a result, the role of endogenous reserves have likely been overestimated in most cases. Stable isotope measurements of endogenous reserves and egg components can help to trace quantitatively the relative allocation of endogenous vs. exogenous reserves to eggs in cases where local (breeding season) diets differ isotopically from endogenous reserves typically acquired on the wintering or staging grounds prior to arrival to breed. Fortunately several avian species change ‘isoscapes’ when travelling to breed and so are amenable to isotopic tracking of their body nutrients to eggs. For example, birds wintering in coastal marine biomes and travelling to terrestrial or freshwater biomes experience a general depletion in the isotopes of several elements (C, N, H, O and S) in their local foodwebs. Previous captive studies have allowed us to estimate the isotopic (δ13C and δ15N) fractionation factors between diet and several egg components (lipid-free yolk, albumen, yolk lipid, shell carbonate) for herbivorous and carnivorous ‘income’ breeders. Currently, we are using the carnivore model to also estimate fractionation expected between endogenous reserves and eggs. The net result of this work has been an isotopic multi-source mixing model to predict the relative allocation of endogenous vs. exogenous protein and lipid capital to eggs. While refinement is needed for this model, it provides a firm basis for future avian isotopic tracking studies involving nutrient allocations.


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