Palaeocolour: A History and State of the Art 11 Fiann Smithwick and Jakob Vinther Alongside being the most diverse and widespread of all tetrapod clades, birds show the most remarkable range of colours in any group of ter- restrial vertebrates (Baker and Parker 1979; Hill and McGraw 2006a). From the cryptic browns and greys of dunnocks and sparrows to the glori- ous rich palette of colours in the birds of paradise, parrots and hummingbirds, almost every colour imaginable to the human observer appears to be present in extant birds (Hill and McGraw 2006a, b). Thanks to features such as tetrachromacy and ultraviolet (UV) vision, many birds can also see colours invisible to di- or tri- chromatic mammals (Vorobyev et al. 1998). Col- our plays a key role in ecology. Be it for camouflage from predators or prey, sexual dis- play, species recognition, or as warning signalling (aposematism), colour has doubtlessly been a key driver in the evolution of bird plumage (Baker and Parker 1979; Hill and McGraw 2006a; Vinther 2015a). The range of colours exhibited by birds can likely be explained, at least in part, by the importance of visual cues in avian signal- ling due to their excellent tetrachromatic visual capabilities (Vorobyev et al. 1998; Koschowitz et al. 2014). These are only rivalled in vertebrates by the equally colourful teleost fishes (Cuthill 2006). Bird colouration has fascinated naturalists and scientists for centuries and helped to galvanise the theories of evolution by both natural and sexual selection (Baker and Parker 1979; Darwin 1859, 1871). The dazzling array of colours seen in birds has traditionally been attributed to two mechanisms of colour production: the utilisation of pigments, biopolymers that differentially absorb and reflect specific wavelengths of light, and nanostructural arrays within feathers (McGraw et al. 2005; McGraw 2006a, b, c; Prum 2006). Structural colours are produced in two primary ways in bird plumage. Iridescence is angle-dependent refraction, most often associated with pigment layers and keratin interacting with incident light to modulate it resulting in the reflection of spe- cific wave lengths of light (McGraw 2006b; Prum 2006; Igic et al. 2016). Non-iridescent structural colour is produced by a complex network of quasi-ordered air bubbles inside the keratin matrix (Babarović et al. 2019). This serves to scatter certain light waves, while an underlying melanin layer usually serves to absorb the remaining unscattered light (Prum 2006). The hues, saturation and brightness of pigmen- tary colours are also controlled in part by their arrangement within the keratin matrix. Thus, a likely continuum in colour production involving both structural and pigmentary mechanisms, rather than a strictly dichotomous division of pigmentary and structural production, exists (McGraw et al. 2005; McGraw 2006b; Prum 2006; Galván and Solano 2016). Novel nuances of bird colour production are still being F. Smithwick · J. Vinther (*) School of Earth Sciences, University of Bristol, Bristol, UK e-mail: Jakob.Vinther@bristol.ac.uk # Springer Nature Switzerland AG 2020 C. Foth, O. W. M. Rauhut (eds.), The Evolution of Feathers, Fascinating Life Sciences, https://doi.org/10.1007/978-3-030-27223-4_11 185