Some bird species, such as the great-crested grebe, hunt underwater for fish and have red eyes. The red colouring is presumably beneficial to these diving birds, but in what way? If it does provide an advantage, why have other birds with similar habits not evolved red eyes?
鈥he red eye is caused by the colour of the iris, which controls the diameter of the pupil. Only some diving birds have red irises.
The pigmentation of the iris is just a device for making the iris as opaque as possible to accurately control the amount of light reaching the retina, by ensuring no light leaks through the iris muscles. Its colour is in this sense irrelevant.
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But eye colour was extensively reviewed by ophthalmologist Ida Mann in 1931, who concluded that iris colour is used as a signal between animals. There has been no evidence to counter this conclusion since.
So that a structure used for signalling is conspicuous, it needs to contrast with its surroundings, and so bright iris colours often contrast strongly with the feather or skin of the head in birds (or, in certain cases, they match it).
Eye colour varies widely. There are grebes with yellow eyes, penguins with black, red or yellow eyes, and cormorants with blue, green, red or black eyes.
Iris colour changes with age in many birds, with the young showing only browns and blacks and not attaining the full bright colour until adulthood. Presumably this is when they need to use iris colour as a signal of fitness or when emotional state becomes important.
Graham Martin, Emeritus Professor, Centre for Ornithology School of Biosciences, University of Birmingham, UK