I can think of a few very different designs that could come of, essentially, anti-gravity birds.
Since wings are not needed to generate lift, they become simply rudders. The wings would be there only as guidance, not as lift, which means they would be fairly stubby. Of course, water is thicker than air, so the wings would probably still need feathers to catch the air, but the wings themselves would be stubby and thick, used for quick bursts of forward motion and direction changes. As penguins hunt fish, these birds would hunt other birds (see "bubble birds" below).
Falcons are fast. They can bunch up their wings and bodies to achieve astounding speeds using nothing but gravity. But if they can zoom up and down with a thought, why not use both directions to increase speed? Their wings would extend backwards as they become more sail-like, essentially becoming a flying wing. With rapid upward and downward motion, wind force would shove them forward at immense speed; I imagine the upper speed limit would be near the speed of sound. They would also grow denser bones, to survive impacts at those high speeds. These birds could hunt anything on land or in the air.
Bubble Birds (aka Flying Chickens)
While some birds are in the air to hunt, other birds only take to the skies to move location. In those cases, wings are only useful for staying up; without a real need for wings, they eventually become tiny fluttery stumps, like the fins on a seahorse. They use their innate hover ability to rise and fall, riding air currents. They stay out of danger by simply floating away, or dropping out of the sky. They probably eat berries and seeds, or small ground bugs.
Birds that eat bugs need speed, but not stopping power. They're fast, but don't need huge muscles to rip their prey apart. Instead, these bird will evolve longer, thinner bodies, with long, fluffy plumage. Rather than flapping, they snake back and forth, zipping forward and turning on a dime. They eat small flying bugs.
Finally, larger predators use their body weight and sharp talons to knock their prey over and gut them, hunting animals as large as goats and sheep. Without a need to balance flight and size, these birds will become absolutely massive. I mean huge. They may not have big wings, but they will be dense as rocks. Think flying turtle dense. Eventually, they will have enough weight that they can simply plummet from the sky and crush whatever creature is below them. Thus the name. First, a shadow; then, THUD, a smashed deer. Or cow. These birds could get big enough to mush anything and everything, and eat what's left.