The drivers and mechanisms of plumage colour evolution

Birds are one of the most colourful groups of animals on the planet. Their colouration has understandably fascinated scientists for decades and this diversity provides opportunities for exploring some fundamental evolutionary questions. In this thesis, by focusing on particular bird groups, I aim to broaden our understanding of the mechanisms and drivers of plumage colour evolution. In the first part of my thesis, I explore behavioural and environmental factors that shape plumage colour evolution in the clade Coraciiformes. I find that, in this clade, the majority of plumage colour variation is explained by variation in environmental light conditions among species, while behavioural traits have limited influence. In the second part of my thesis, I test a previously described hypothesis for an evolutionary pathway between grey and blue plumage colouration. Using phylogenetic models of trait evolution, I find support for this hypothesis, confirming a macroevolutionary pathway towards colour blue from colour grey via colour slate in the clade Thraupidae. Finally, in the last part of my thesis I studied variation in feather nanostructure [specifically the keratin and air matrix (spongy layer) within feathers] to identify structural elements underpinning the described evolutionary transition between grey and blue. In the case of blue colour in Tanagers, I find that it is variation in many elements of the spongy layer that explains colour variation, rather than the presence or absence of any of its elements. Together, the results of this thesis highlight the importance of environmental factors in driving plumage colour evolution, as well as mechanistic changes (and associated developmental constraints) involved in the evolution of novel colour phenotype.