Iris coloration in birds (particularly passerines) is a topic that has intrigued me ever since I started observing the behaviour of different starling species. Some years ago in this publication, Peter Ryan provided an excellent review of the structure and function of birds’ eyes (African Birdlife September/October 2013: 28‒34). My focus here is much narrower, but let’s start with some basic anatomy.
As in humans, the pigmented area of a bird’s eye is the iris, which surrounds the pupil, but the sclera (the white of our eyes) is not visible when we look at the eye in a living bird. Pupil size in animals varies primarily with light conditions and is influenced by internal physiology. However, whereas in our eyes variation in pupil diameter is an automatic reaction controlled by the sympathetic nervous system, birds also have voluntary control over muscles in the iris. So deliberate changes in pupil size can display either a very large area of pigmented iris or a very small area, under exactly the same light conditions. Clearly this can serve as a close-range signal during interactions between individual birds.
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