Seeing in the ultraviolet
Even in the visible spectrum, birds can discriminate more subtle color distinctions than we can, thanks to their at-least-five functional cone photoreceptor types (we only have three). But it’s in the ultraviolet (UV) part of the spectrum where they literally can see what we can’t.
Somewhat disappointingly, birds don’t generally have secret UV patterns the way that, for example, some flowers do (Andersson 1996). Instead, they seem to use UV to augment signals we can already see: bluebirds turn out to reflect UV, as do the spots on some thrushes, and so on. But the UV can still contain information invisible to our eyes. In the Alpine Swift and the European Starling, better-fed chicks reflect more UV from their skin; their parents can use this information to give more food to scrawny chicks in good times, or to cut their losses and favor the healthiest chicks in lean times (Bize et al. 2006).