Evolutionary dynamics of pigmentary grey and non-iridescent structural blue colouration in Tanagers (family: Thraupidae)

Birds are one of the most colourful animal groups in the world and there are multiple ways by which they achieve this feature. Mechanisms of colour production range from pigmentary (pigment deposition) to structural (nanostructural arrangements), or the combination of both. Despite the huge breadth of colour gamut, the basic components of feathers are shared across all of them (keratin, air and presence of pigments in accordance with the colour produced). It has been shown that in some instances, colour evolution between pigmentary and structural colours can proceed by rearrangement of the nano-structural elements of feathers. Here, we investigated evolutionary transitions between pigmentary grey and non-iridescent structural blue. We focus on the Thraupidae (tanagers and allies) that display a variety of blues and greys including a potential transition state that we refer to as slate. We used digitally calibrated images of birds to quantify colour and determine the distinctiveness of slate colour in colourspace. Following, we identify the most likely pathway for the evolution of the colour blue: from grey via slate colour. Our research reveals a new pathway in the evolution of blue colour. ### Competing Interest Statement The authors have declared no competing interest.