Rods and cones are the two kinds of light receptors in a bird's eye. Rods are sensitive to small quantities of light so they are better for night vision. Cones are better for day vision as they are sensitive for large amount of light and can detect specific wavelengths of light (colors).
Generally cone cells are sensitive to primary color wavelengths of light i.e. red, green and blue. But birds have extra pigment in cone cells, that's why they are tetrachromatic i.e. possessing UV or ultraviolet sensitivity along with red, green and blue.
Now, when the pigment absorbs light, retina changes its shape and alters the membrane of the cone cell affecting neurons. In this way, the neurons possess information from several photoreceptor cells, which in turn triggers a nerve impulse that relays along the optic nerve to specialized visual centers in the brain.
The unique feature of cone cell that exists in birds is that, each cone cell consist of a colored oil droplet which is placed in such a way that light passes through it before reaching the visual pigment. The oil droplets act as filters by narrowing the absorption spectra of the pigments that reduces overlapping between pigments and increases number of colors that a bird can discern. Six cone cell droplets have been identified.