Why is the Robin’s breast red? Why are any of the parts of any birds colorful? To make it easier for birders to identify them, of course!
But seriously, Science has a more interesting set of answers, and some recently published research on European Robins helps to examine this question in some detail.
There are several reasons that scientists have postulated for any kind of signaling seen in animals, and bird’s colors are clearly some kind of signal. Here’s a short list of them: