How do porphyrins create red flecks on some bird eggshells?

The red flecks observed on the eggshells of certain birds, such as plovers, are created by the presence of a specific porphyrin pigment known as ooporphyrin.1

Porphyrins are a class of nitrogenous biological pigments characterized by a complex cyclic structure consisting of four pyrrole rings linked by methine groups. In the context of avian biology, these pigments are deposited onto the eggshell, resulting in the characteristic reddish coloration or spotting patterns. While other pigments like biliverdin—which is derived from the breakdown of porphyrin—are also responsible for various eggshell colors, ooporphyrin specifically accounts for the red flecks found in many species.12