The colour that gives the blue penguin its name is produced by a protein nanostructure in the bird's feathers.
Matthew Shawkey at the University of Akron in Ohio, Vinodkumar Saranathan at Yale University in New Haven, Connecticut, and their colleagues analysed barbs on the feathers of blue penguins (Eudyptula minor; pictured). They found that a quasi-ordered structure made up of densely packed, parallel nanofibres of the protein β-keratin scatters light in a way that produces the blue tinge.
The structure is similar to ones made of collagen that are responsible for colours in the skin of some birds and mammals. Only two other non-iridescent colour-producing nanostructures have previously been reported in birds, the authors say, and both derive from β-keratin.
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Nanofibre makes penguins blue. Nature 470, 309 (2011). https://doi.org/10.1038/470309c