Variation in the distribution of pigment within the shell structure of birds' eggs



Very little comparative study has been made of the distribution of pigments within the structure of the egg-shell. The present paper is a general account of the distribution of the pigments within the thickness of the egg-shells of the different avian genera, and was undertaken in order to assess the value of this character for taxonomic purposes. Egg-shells tend to cleave along a radial plane and the radial surfaces of shell fragments were examined under a microscope and the distribution of colours noted.

Only two pigments are known to be present in most eggs, a blue biliverdin and a protoporphyrin responsible for reds, browns, blacks and yellows. The distribution of colours as it appears in fragments of egg-shells of the different avian families is described, those of the Passeriformes being discussed as a single group. In general, the shells appear white or blue, with other pigment present either as a thin superficial layer, or in small patches scattered at intervals through the thickness of the shell. In 11 genera, various pigments were found to be present through a significant portion of the outer shell structure; from one-third to one-half the total thickness. There did not appear to be any common factor underlying this occurrence.

The only major variation in distribution was found in patterned eggs, some of which had all the pigment producing the pattern limited to the surface layer of the shell, while in others this was present at varying depths within the thickness of the shell. The former type was found in eight families only, and in three of these, Falconidae, Opisthocomidae and Jacanidae, was limited to one family in an order containing numerous families.

Fugitive pigments occur in some shells. An unstable porphyrin gives a pink flush only seen in very fresh eggs. In a few species the fresh eggs are green but subsequently become blue. The green colour appears to be due to a combination of blue biliverdin and a yellow pigment, possibly bilirubin.

In view of the general similarity of pigment distribution in most egg-shells this character would appear to have limited use in taxonomy, but the difference in distribution of pattern pigments might be of some significance.