While many birds have iridescence that is visible to humans, many also have colorful markings visible under ultraviolet light.
Via Atlas Obscura:
Studies have shown that seeing in UV helps birds with all kinds of tasks, from finding food to differentiating their own eggs from those of nest parasites. Some, like grouses, use it to more effectively forage for bilberries, which start reflecting UV light when they're ripe. Others, like kestrels, may use it to track voles, which apparently mark their paths with bright yellow trails of UV-saturated urine (although this finding has been subjected to recent scrutiny).*
It works on common pet birds like budgies and pigeons, too!
Image: YouTube / Silence