Oxygen and carbon isotope compositions of fossil bird eggshell calcite (δ¹⁸O_calc and δ¹³C_calc) are regularly used to reconstruct paleoenvironmental conditions. The δ¹⁸O_calc values mainly reflect the oxygen isotope composition of the waters ingested by birds (δ¹⁸O_w), which in turn are controlled by local climatic parameters (air temperature, local humidity), whereas the δ¹³C_calc value provides information on birds foraging environment as well as local relative humidity. However, the interpretation of δ¹⁸O_calc values of fossil eggshells is limited to qualitative variations in local climatic conditions as oxygen isotope fractionations between calcite, body fluids and drinking water have not been determined yet. For this purpose, eggshells, albumen and drinking water of extant birds have been sampled and analyzed for their oxygen and carbon isotope compositions. Enrichments in ¹⁸O relative to ¹⁶O between body fluids and drinking water of +2.2±0.4 ‰ for semi-aquatic birds and of +4.1 ±0.5‰ for terrestrial birds are observed. Using our dataset and published values, two empirical equations relating the δ¹⁸O_calc value of eggshell calcite to the δ¹⁸O_w value of ingested water have been established for terrestrial and semi-aquatic birds. These equations have been applied to two cases of fossil eggshells from Lanzarote, Canary Islands, in order to infer the ecologies of the Pleistocene marine bird Puffinus sp. and of the enigmatic giant birds from the Pliocene of Valle Grande. Both δ¹⁸O_calc and δ¹³C_calc of Puffinus eggshells point to a semi-aquatic marine bird ingesting mostly seawater with a δ¹⁸O_w value -0.6±0.3‰. In the case of the Pliocene giant bird, low δ¹³C_calc values of eggshells point to a terrestrial lifestyle and the δ¹⁸O_calc values allowed us to calculate an ingested local freshwater δ¹⁸O_w value of -2.8±0.3‰.  This set of equations can help to quantitatively estimate the origin of waters ingested by extinct birds as well as to infer either local environmental (marine or freshwater) or climatic (local air temperatures, precipitations) conditions.

Authors : N. Lazzerini , C. Lécuyer, R. Amiot, D. Angst, E. Buffetaut, F.  Fourel , V. Daux  et al.

Ref. :  Sci. Nature 103:81