While looking at some sparrows recently, I noticed a few flashing white outer tail feathers as they flew away from me. I used this feature to identify dark-eyed juncos and vesper sparrows in that flock. A little later I startled a Northern flicker off the ground and watched it flash a conspicuous white rump as it flew away. Still later, a Northern mockingbird flew close as I made squeaky noises to attract some birds. It perched and slowly raised its wings to expose large white patches. When it finally identified me, it flew away, flashing bright white outer tail feathers, too.
White is a color that is often used in nature to communicate danger or a warning to anything that may be paying attention. Think about when a cottontail rabbit or a white-tailed deer is startled or runs away from you. They too flash bright white as they depart.
In the case of the mockingbird, the slow raising of the wings to show off the white patches is a defensive sign and a warning to other birds that they are intruding on its territory. The sparrows and flicker flashing white while they flee are communicating to other members of their flock that there is an imminent threat. It also provides a beacon for the other birds to follow as they all try to escape the threat.
There are many other local bird species that have varying amounts of white in the tail. Eastern towhees, American pipits, Eastern meadowlarks, pine warblers, horned larks and longspurs all have it. Many waterfowl, woodpeckers, white-rumped sandpipers, greater and lesser yellowlegs, black-bellied plovers and loggerhead shrikes, to name just a few more, show white wing or rump flashes. Again, these white areas are best seen when the bird is flying away from you.
A glimpse of a flash of white in a flushed sparrow flock quickly confirms the presence of vesper sparrows or juncos. Birders use white rumps and white wing patches to quickly identify birds in flight, sometimes at great distances.