By far the most abundant cone pigment in every bird species examined is the long-wavelength form of iodopsin, which absorbs at wavelengths near 570 nm.

Within the eye, 11-cis-retinal is bound to protein "opsin" to form rhodospin in rods and iodopsin (cones) at conserved lysine residues.

Rhodopsin, of rods, breaks down into opsin and retinal; iodopsin of cones breaks down into photopsin and retinal.

The pigment, called iodopsin or rhodopsin, consists of large proteins called opsin (situated in the plasma membrane), attached to a covalently bound prosthetic group: an organic molecule called retinal (a derivative of vitamin A).

The rhodopsin or iodopsin in the disc membrane of the outer segment absorbs a photon, changing the configuration of a retinal Schiff base cofactor inside the protein from the cis-form to the trans-form, causing the retinal to change shape.