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Distribution of the different oil droplet types of birds (adapted from fig. 9 of [9]). The diagram shows a schematic bird eye with the four single cone classes with their oil droplet types (T, transparent; C, clear (or colourless); Y, yellow; R, red), and the double cones with P (pale) droplets (after fig. 8 of [2]). Inset: section of the retina of an ostrich (Struthio camelus); scale bar, 10 µm (adapted from [12]).
