Drab cuckoos retain the melanosome types of brilliantly coloured relatives

Structural colour is widespread in animals, yet we know relatively little about its evolution and development. We shed light on the evolution of structural barbule colour (often referred to as iridescent structural colour) by describing feather nanostructures responsible for plumage colours in the cuckoos (family Cuculidae). The melanosomes found in feathers with structural barbule colour have specialized shapes: hollow rods, thin solid rods, hollow platelets or solid platelets. In contrast, it is often assumed that drably coloured feathers possess thick, rod-shaped melanosomes. We show that this assumption is unfounded in cuckoos. We describe structural barbule colour in the plumages of 126 cuckoo species and map its phylogenetic distribution. This reveals that structural barbule colour is widespread in cuckoos but has probably been lost several times. We then use transmission electron microscopy to describe the feather nanostructures of 21 cuckoo species. Surprisingly, the drab feathers of many cuckoo species contain melanosomes with specialized shapes. Thus, specialized melanosome shapes can be retained in the plumages of drab species, potentially making it easier for structural barbule colour to evolve again in the future. This discovery supports the idea that evolutionary history plays a key role in shaping the evolution of plumage colour diversity.