Iris (plural irides)The coloured part of the eye surrounding the pupil and controlling the latter's size by expanding or contracting according to light intensity. The colours of irides in birds vary, being, for example, yellow in the Goldeneye Bucephala clangula and bluish in the Jackdaw Crovus monedula.
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From Peter Weaver's Birdwatcher's Dictionary
Copyright © 1981 by Peter Weaver