Eggshell structure in the Shoebill and pelecaniform birds: comparison with Hamerkop, herons, ibises, and storks

Scanning electron microscope investigation of eggshell structure supports earlier beliefs that the pelecaniform family assemblage is a natural group. All families of pelecaniform birds except the Phaethontidae show complete similarity in the structure of both the crystallized eggshell and the accessory (amorphous) material (cuticle and cover) located in the outer surface of the shell. Comparisons with other avian taxa (93 families of non-passeriform birds) show that in terms of eggshell structure, Scopus umbretta is an "advanced" ardeid and Balaeniceps rex a rather unusual pelecaniform. All these birds should form one large group with the Ardeidae and clearly distinguished from the Ciconiidae. The results of this investigation imply that, ultimately, the deposition of microglobular material in the cover of avian eggshell is subject to phylogenetic constraints rather than having an adaptive (ecological) cause.