Porphyrins are deposited in growing feathers and fluoresce under ultraviolet illumination. They degrade with light exposure, so a feather grown this year glows brightly while one retained from a previous year glows faintly or not at all.
Saw-whet owls replace their flight feathers in a partial pattern, keeping some from earlier years. Under a portable ultraviolet lamp the underwing shows bands of bright and dull, and the arrangement of those bands corresponds to known moult sequences. Banding stations use the technique routinely alongside plumage and eye colour to assign birds to age classes, which is otherwise difficult in a species with little visible age variation.
The method has limits worth stating. Fluorescence intensity depends on the lamp, on how much sun the bird has had, and on the observer's judgment, so it is a supporting criterion rather than a definitive one. The same porphyrin fluorescence occurs in other owls, so the technique is not unique to this species.