When a Northern Flicker takes flight, a bold patch of white feathers flashes on its rump, in contrast to its brown body. This white rump likely evolved as an anti-predator adaptation. A hawk flying in pursuit of a flicker may focus on the white spot rather than the darker image of the whole bird. This gives the flicker an advantage as it dodges into the trees. Then the white target disappears beneath the bird’s folded wings. An instant of confusion for the hawk allows just enough time for the flicker to sidle around to the other side of the tree.
https://www.birdnote.org/listen/shows/flickers-white-rump

Understanding how the wings can control the visibility of the rump patch makes clear that it is no coincidence that a brightly colored rump is a common feature of many species. It’s a valuable signal, available when it’s needed but hidden at will. Of course, the bright rump patch becomes very visible when a bird takes off, and this also has some benefits. Research has shown that a sudden flash of color in the instant before a predator strikes can cause the predator to flinch, possibly allowing the prey to escape.

https://www.birdwatchingdaily.com/birds/david-sibleys-id-toolkit/how-to-use-birds-rump-color-as-a-field-mark/