The phosphate-rich cover on the eggshells of grebes (Aves: Podicipitiformes)

Spheres of a nut-like morphology–a "kernel" formed from spherules contained in a "shell" of globular sub-units–were a unique feature of the cover on the eggshells of six species of the order Podicipitiformes: Red-necked grebe (Podiceps grisegend) , Great crested grebe (Podiceps cristatus) , Little grebe (Tachybaptus ruficollis) , Black-necked grebe (Podiceps nigrkollis) , Slavonian grebe (Podiceps auritus) and Pied-billed grebe (Podilymbus podiceps). The spheres did not give a pattern with X-ray diffraction; the innermost ones were embedded in the dense crystalline layer on the outer surface of the calcitic portion of the shell. Electron-probe microanalysis revealed that the cover of spheres contained principally oxygen, calcium, phosphorus and sulphur and infra-red analysis demonstrated the presence of phosphate. The outer surface of the stratum of spheres was bounded by a thin, fissured layer of amorphous material.
These observations raise questions about the ionic environment obtaining in the shell gland towards the end of shell formation and the adaptive significance of the cover on the eggshells of members of the Podicipitiformes.