The pattern of distribution and interaction of metals and calcium in eggshells and egg contents in relation to the embryonic development of eggs in a small passerine bird

Differences in element concentrations have been observed between embryonated and non-embryonated eggs of the Eurasian Reed Warbler Acrocephalus scirpaceus, presumably due to the resorption of calcium and other elements from the shell. To investigate this difference, we tested the hypotheses that (1) the distribution of elements in eggshells and egg contents, (2) the individual element-element relationships between shells and contents, and (3) the relationships among the elements in eggshells and egg contents vary between the two egg samples. Based on the levels of 11 elements (Cr, Cu, Ni, Cd, Pb, Mn, Fe, Co, Zn, Mg and Ca) in Reed Warbler eggs, our results confirmed the second and third hypotheses. Importantly, we found that embryonic development tended to blur the lines between two types of relationships: (1) between elements in eggshells and egg contents through an increase in the number of significant correlations, some of which may be enhanced by high levels of certain elements (Cd, Co, Pb, Zn are indicative of trace metal pollution in fish ponds); (2) among the various elements in eggshells and egg contents, as demonstrated by the differential grouping of elements in principal components analysis (PCA). The consistent positive direction of relationships for almost all elements in both the shells and contents of eggs is presumed to result from the initial parallel co-sequestration of all the elements which accompanies intensive calcium transfer from the female body into the incipient egg. Our study highlights the fact that only limited inferences can be drawn from the interactions of metals and calcium in eggshells and egg contents if the fertility status of the eggs is unknown.