Cassowary gloss and a novel form of structural color in birds

One of the two lineages of extant birds resulting from its deepest split, Palaeognathae, has been reported not to exhibit structural coloration in feathers, affecting inferences of ancestral coloration mechanisms in extant birds. Structural coloration in facial skin and eggshells have been shown in the lineage, but not feathers. Here, we report the first evidence for two distinct mechanisms of structural color in palaeognath feathers. One extinct volant clade, Lithornithidae, shows evidence of elongate melanin-containing organelles uniquely associated with glossy/iridescent color. This mechanism of structural color is present in fossil outgroups and neognath birds. Second, we demonstrate a structural basis of exceptional gloss in extant cassowary feathers. Gloss is proposed to be an intermediate phenotype between matte and iridescent plumage, conferred by a thick and smooth feather rachis. Rachis-based structural color has not been previously investigated. The new data inform relationships between avian melanin-based coloration and feather structure.