In many birds, the color of the iris differs between age and sex groups. The red eye of an adult Spotted Towhee or Gray Catbird, for example, is muddy brown in juveniles, and the bright white eyes of female Bushtits distinguish them readily from males.

An unusual sexual difference is shown by Blue-footed Booby. Both sexes have strikingly pale irides, but in females, the inner iris has a dark ring, making the pupil look noticeably larger than in males.

Blue-footed Booby. Photo: Rick Wright

There’s probably no better place in the world to enjoy up-close encounters with this species than the Galapagos islands, where confiding birds can be seen in large numbers. Join Rich Hoyer on his next charter cruise aboard the luxurious M/Y Integrity.