Hi Paul, I am in regular contact with Hein van Grouw, Senior Curator, Bird Group at the Natural History Museum, so I e-mailed him. This is his reply:
Thanks for your email. Yes, wood pigeons seem to have non-circular pupils. The pupil, however is circular but due to a spot of dark pigment in the iris the pupil seems to be non-circular. I have no idea what the reason (advantage) is for this. What I do know is that several other (tropical) species, mainly Fruit Doves, have it too.
Just how old are you Gav ( still at school?)Everyday is school day
Many animals have irises that are not round: cats have slits, goats have squares. The reason? An evolutionary advantage selected for that shape.