Some birds use fast size change of their pupils as a way to communicate. At least in (pet) parrots, there seems to be an English term for this: „eye pinning“ or „eye flashing“. I captured the immense speed of dilation/constriction in an aroused Woodpigeon / Ringeltaube (Columba palumbus). The male was actively displaying at a female and seemed to use its flashing eyes as a part of the courtship ritual. It was striking how tiny the pupils got and how they widened to more regular size within fractions of a second. Light conditions remained constant during the observation.
Birds have greater control over their pupillary response than mammals, and some birds such as crows, starlings and parrots are known to change the size of their pupils in social interactions.African Birdlife, September/October 2013
Look at the size of the black pupils, don’t get distracted by their apparently non-round shape. Like some other bird species (i.e. Black Woodpecker, Oystercatcher and a number of pigeons), Woodpigeons have a pigmented spot close to the pupil that seemingly „merges“ with it, giving it an odd shape.