Egg pigmentation probably has an early Archosaurian origin

Biogeographic history, egg colouration, and habitat selection in Turdus thrushes (Aves: Turdidae)

Despite broad interest and recent experimentation, there is no single ecological model accounting for the adaptive significance of the diversity of avian eggshell colouration. The often blue-green eggs of Turdus thrushes are a charismatic example of this, having long captured cultural and scientific attention. Although the biology and evolutionary history of "true" thrushes is well understood, little is known about correlated evolution between shifts in habitat and eggshell pigmentation, and how these shifts map with Turdus biogeography. We applied phylogenetic comparative methods to assess the evolutionary timing of divergence and variation of life history traits and eggshell colouration and maculation presence in the genus. We found that eggshell colour diversified independently on several occasions in the past 11 million years, with much of the variation occurring within the last 4 million years. The majority of Turdus species lay blue-green eggs and also tend to be sedentary and forest-dwelling. Diet generalist species and species which have transitioned to a forest habitat are more likely to lay white eggs (10% of studied species). In turn, lineages in any habitat were more likely to transition to blue-green eggs. We found that variation in egg colour is increased in some clades, of which two lineages radiated in South America and the East Palearctic, in the past 2-4 million years. These findings provide support for the hypothesis that white eggs are more conspicuous to predators in open environments and that multiple, non-mutually exclusive constraints operate on the adaptive function of avian eggshell colour.

Avian Coloration Genetics: Recent Advances and Emerging Questions

The colorful phenotypes of birds have long provided rich source material for evolutionary biologists. Avian plumage, beaks, skin, and eggs-which exhibit a stunning range of cryptic and conspicuous forms-inspired early work on adaptive coloration. More recently, avian color has fueled discoveries on the physiological, developmental, and-increasingly-genetic mechanisms responsible for phenotypic variation. The relative ease with which avian color traits can be quantified has made birds an attractive system for uncovering links between phenotype and genotype. Accordingly, the field of avian coloration genetics is burgeoning. In this review, we highlight recent advances and emerging questions associated with the genetic underpinnings of bird color. We start by describing breakthroughs related to 2 pigment classes: carotenoids that produce red, yellow, and orange in most birds and psittacofulvins that produce similar colors in parrots. We then discuss structural colors, which are produced by the interaction of light with nanoscale materials and greatly extend the plumage palette. Structural color genetics remain understudied-but this paradigm is changing. We next explore how colors that arise from interactions among pigmentary and structural mechanisms may be controlled by genes that are co-expressed or co-regulated. We also identify opportunities to investigate genes mediating within-feather micropatterning and the coloration of bare parts and eggs. We conclude by spotlighting 2 research areas-mechanistic links between color vision and color production, and speciation-that have been invigorated by genetic insights, a trend likely to continue as new genomic approaches are applied to non-model species.