Figure 1.

Anatomy of spongy medullary barb tissue. (a) Image of an eastern bluebird S. sialis (photo by Mark Liu) showing feathers with blue colour created by coherent scattering of light from spongy tissue in feather barbs. (b) SEM image of the outer surface of blue feather barbs illustrating the dimensions (x,y,z) examined in this paper. (c) Two-dimensional TEM image of a transverse section through a single feather barb. The x and y planes are illustrated. Moving inwards, the layers shown are the outer keratin cortex (C), spongy medullary layer (SL) and vacuole (V). Cell boundaries (dark lines) can be seen in the upper left section of the cross section. The square inset shows a close-up of the spongy layer.