Egg shells of the Gaviiformes, Procellariiformes, Podicipitiformes and Pelecaniformes have been studied using general inspection, chemical analysis, histological and plastic embedding techniques.
In all four orders there is a true shell consisting of large crystals having their origin at the surface of the membrane. In the Gaviiformes and Procellariiformes, the true shell is then covered with a cuticle, while in the Podicipitiformes and Pelecaniformes (except for the Phaethontidae) there is a calcareous cover over the true shell. Histological studies showed no major difference in the organic matrix of the true shell as between the four orders.
Pore channels are all single and, in those egg shells which have a cover, these channels do not penetrate the cover. Considerable pigmentation is present in the Gaviiformes egg shells and also in the Phaethontidae, and pigment spots occur not only on the surface but also in the body of the shell. Studies of the apparent density of the true shells and the covers showed interesting differences.
The taxonomic aspect of the results is discussed.