Hi Bill,

perhaps the following literature citation is of help.
Greetings

Roland


AU HOBSON, KA
TI RECONSTRUCTING AVIAN DIETS USING STABLE-CARBON AND NITROGEN ISOTOPE
   ANALYSIS OF EGG COMPONENTS - PATTERNS OF ISOTOPIC FRACTIONATION AND
   TURNOVER
SO CONDOR
AB Because eggs are composed of nutrients that are ultimately derived from
   the diet of adult females, the relative abundance of
   naturally-occurring stable isotopes of carbon and nitrogen in eggs
   should be related to those in their diet and this may form the basis of
   a method for tracing diets. Before such dietary reconstructions can be
   established, however, it is necessary to know how isotopic signatures
   change (or fractionate) from the diet during the synthesis of various
   egg components. In this study, stable-carbon and nitrogen isotopic
   fractionation factors between diet and whole yolk, lipid-removed yolk,
   albumen, and shell membranes of eggs of captively-raised Mallards (Anas
   platyrhynchos), Japanese Quail (Coturnix japonica), Prairie Falcons
   (Falco mexicanus), Peregrine Falcons (Falco peregrinus), and Gyrfalcons
   (Falco rusticolis) were established. In addition, carbon isotope
   fractionation patterns between diet and eggshell carbonate and yolk
   lipid were determined for quail, Mallards and falcons. On average,
   yolk, albumen and membranes were enriched in N-15 relative to diet for
   all birds by 3.4 parts per thousand, a value typical of processes
   associated with protein synthesis. For quail and Mallards, albumen,
   membranes, and shell carbonate were enriched in C-13 relative to diet
   by 1.5, 3.6, and 14.9 parts per thousand, respectively, and whole yolk
   was depleted in C-13 by 2.6 parts per thousand due to the presence of
   yolk lipids. Falcons showed lower carbon diet-tissue fractionation for
   all tissues (albumen: +0.9, whole yolk: -1.9, membranes: +2.7,
   carbonate: +11.2 parts per thousand) and this may be due to a greater
   reliance on dietary lipids vs. carbohydrates for the synthesis of egg
   components or on fractionation differences associated with fermentation
   of carbohydrates. Patterns of isotopic fractionation between diet and
   Mallard eggs were not influenced by clutch order. A diet-switch
   experiment using quail indicated that albumen, shell membrane and shell
   carbonate values reflect diet integrated over 3-5 days and yolk over
   eight days prior to laying. Because birds were fed ad libitum, these
   data provide baseline data on fractionation patterns for birds
   optimally mobilizing nutrients from diet to egg components. These
   values may differ for those wild birds that rely more heavily on
   endogenous reserves.
PD AUG
PY 1995
VL 97
IS 3
BP 752
EP 762




William Patterson wrote:

> Dear Isogeochemists,
> Has anyone conducted isotopic analyses on carbonate eggshell protein 
> and lipids that have separated the two components. How do the isotope 
> ratios of protein and lipids change in older (fossil and subfossil), 
> and how does that diagenesis (if any) affect the C/N ratios of the 
> bulk organic matter.
>
> Any advice, knowledge, or references would be greatly appreciated!
> Cheers,
> Bill