Vision is the most important sense for
birds, since good eyesight is essential for safe flight, and this group has a number of adaptations which give visual acuity superior to that of other
vertebrate groups; a pigeon has been described as "two eyes with wings". The avian eye resembles that of a
reptile, with
ciliary muscles that can change the shape of the
lens rapidly and to a greater extent than in the
mammals. Birds have the largest eyes relative to their size in the animal kingdom, and movement is consequently limited within the eye's bony socket. In addition to the two eyelids usually found in vertebrates, it is protected by a third transparent movable membrane. The eye's internal anatomy is similar to that of other vertebrates, but has a structure, the
pecten oculi, unique to birds.