A gimbal head for your tripod is an essential tool for capturing fast-moving birds in flight with a heavy camera and long lens.
Birds in flight move at tremendous speeds. Small songbirds like wrens and sparrows fly 10 to 20 miles (16 to 32 kilometers) per hour, while ducks, geese, and pigeons can fly at speeds up to 60 miles (97 kilometers) per hour. Some birds, like the peregrine falcon and golden eagle, have been timed diving at speeds exceeding 170 miles (274 kilometers) per hour. 
Getting good action shots requires nimble equipment. Somehow “nimble” and a 12 pound 600mm f4 lens with a four pound professional camera attached don’t seem to go together.
Fortunately, there are some tools you can buy that will help even the odds. For years, I struggled to mount heavy cameras and lenses on a traditional tripod head mounted beneath the lens. The problem: the camera and lens get too “tippy” the moment you point up or down. You’re always fighting with a runaway camera/lens combo. Then, I discovered a marvelous tool called the Wimberley head. It has a gimbal mount design. (See pic below.)
gimbal head is a bit pricey but it’s worth every penny in my opinion. It makes the difference between getting and missing the shot. Why spend thousands on great cameras and lenses if you’re going to miss the shot because of a tripod head that wasn’t designed for the task?
The Wimberley gimbal head puts the center of gravity of the camera/lens combination at the exact center of the vertical and horizontal pivot points. This, in essence, makes that 15 pound camera lens combo virtually weightless.
You can move the entire gimbal head, camera, lens and teleconverter combination with a single finger. And when you move it, it stays put without having to lock down the tripod head. The Wimberley L-shaped arm moves up or down so that you can center the weight vertically. And the adapter plates that mount to your lens have horizontal tracks on them so you can shift the center of weight forward or back until everything is perfectly balanced.
Now, instead of fighting your equipment, you’ll find that a gimbal head moves weightlessly and without friction – in a word, effortlessly.
I have no experience with a gimbal head other than the Wimberley. Others may be perfectly satisfactory. But its hard to imagine any gimbal head moving smoother than the Wimberley. It certainly took my game to a new level. I remember thinking the first time I used it that it gave me the best of both worlds – the stability of a tripod and the freedom of shooting handheld. It’s not quite like shooting handheld because you can’t point straight up, but it’s pretty close.