Interspecific differences in eggshell thickness and the elemental composition of pigment spot and plain shell regions in altricial and precocial birds

Our study reveals for the first time in a quantitative manner the differences in eggshell thickness and elemental composition between pigment spots and adjacent plain eggshell regions in bird species from three developmental modes, i.e. altricial (Great Reed Warbler Acrocephalus arundinaceus), semi-precocial (Black-headed Gull Chroicocephalus ridibundus, Mediterranean Gull Ichthyaetus melanocephalus, Sandwich Tern Thalasseus sandvicensis and Black Tern Chlidonias niger) and precocial species (Western Capercaillie Tetrao urogallus and Black Grouse Lyrurus tetrix). From a broader biological/biogeochemical perspective, our data broaden the knowledge in the heterogeneity of elemental composition between adjacent spotted-plain regions of eggshells, previously demonstrated in galliform birds, which differ in colour for many chemical elements. These differences, however, were distinctive of individual species and varied between chemical elements. There emerged a clear and consistent trend of higher elemental concentrations within pigment spots compared to plain eggshell regions, particularly in the case of five elements: C, Pb, Cu, Cr and Al. The most important and novel finding of our study is that even though the eggshells of gulls and terns (semi-precocial species) and of the passerine (altricial species) were thinner in the spotted regions (there was thus less shell material) compared with those of precocials (the eggshells being thicker eggshells at the spots), the spotted regions of all these eggs contained disproportionally higher concentrations of most major and minor chemical elements. Hence, this appears to be a general rule across all birds, regardless of the eggshell thickness in the spotted or plain regions and/or their developmental mode. In other words, this finding confirms that the thickness of the eggshell itself does not affect the concentrations of elements, and that the key issue related to this is the presence of one or more protoporphyrin layers along with the accompanying pool of chemical elements within the spotted eggshell region.