One of the most important factors that affect the quality of images for many photographers using lenses longer than 200mm is camera shake. The longer the lens, the more apparent this problem becomes. No one can handhold a camera without having those small muscle tremors that can affect image quality. Few tripods are truly rock solid.
How do we know we are having a problem with camera movement? If the subject in your image is just a little out of focus but nothing else in the image is in focus either, then camera shake is a real possibility. If you can see ghosting or smearing along contrasty edges in the entire image, then it is camera movement. For birds, one thing I look at is the catch light in the eye. The reflection of the sun should be round and if it is oblong and nothing else either at the same distance or anywhere in the image is in focus, then it is camera shake and not subject movement.
Dealing with camera shake, when handholding, may be as simple as learning to release the shutter properly. Many people in the excitement of the shoot jab the shutter button down causing the camera to jerk up or down. I teach people to place their finger half on and half off the shutter button and to roll their finger over it to release it. You can roll your finger a little to initiate focus and then roll it a little more when you want to take the picture. It is just like firing a rifle: Take a breath and slowly release it as you roll your finger over the shutter release.
Long lenses, especially for handheld birds in flight, require high shutter speeds. The rule of thumb that states that you should use a minimum shutter speed of 1 over the focal length just doesnt work. I like to say that you need to at least double that. Dont forget that with a crop sensor, you need to factor that in. So a 400mm lens on a 1.6 crop body is effectively a 640mm lens. I get my best results with the 500mm on a 7D at 1/2000 and above. Image stabilization (or vibration reduction) is of little value at these higher shutter speeds and with some lenses seems to slow down focus acquisition and tracking. IS/VR was not designed to compensate for the gross, erratic motions of following a bird in flight. However, when the light is waning and we are forced to use longer and longer shutter durations then it is worth a try.
Use proper panning technique with Birds in Flight. For more in depth info, see my previous post here on the BIF/BOW section at: http://www.uglyhedgehog.com/t-88437-1.html
For birds that are more stationary when handholding, find something to brace the camera on. Leaning against a tree, a fencepost, or a wall, for example, can help reduce camera movement. If you are shooting from the car, turn it off. Use a bean bag on the window sill or, if you dont want to spend that kind of money, get a pool noodle, cut it to length, slit it half way through lengthwise and put it on the edge of the partially rolled up glass. Make sure you and anyone else in the car sits still!
Although tripods can really stabilize a camera, really big lenses need really heavy, strong, expensive tripods. Thats great in a stationary setting but who can carry a 20 lb tripod with 15 lbs of camera and lens plus all of our other gear around all day? So we get a lighter weight tripod, or the best we can afford. Even just pressing the shutter button can introduce vibrations. With a long lens and big lens hood the breeze can move things around too. Use a remote shutter release, either cable or radio or use the self timer so you are not touching the camera when the shutter goes off. Many tripods have a hook under the head. This is so that you can tie it down. Hang your camera bag or some other heavy object from it and that will dampen any vibrations. Use Image Stabilization or Vibration Reduction. What? Despite what you may have heard, you CAN use IS with a tripod! The movements induced by a gusty wind are exactly the kind of vibrations that IS is designed to handle. Canon even recommends it. Most modern lenses are tripod aware. Canon lists only 4 lenses in their lineup that are not tripod aware. Unfortunately, this list includes the 100-400mm L zoom that some of us use so be aware of that. Im sure Nikon has similar features. These techniques are doubly important when using long lenses with tele-extenders.
Learn to reduce camera shake using one or more of these techniques and you will be well on your way to getting the best pictures possible out of your investment!
500mm lens at 1/400, f/8. Look at the eye in the download and you will see the catchlight is oblong indicating camera movement. It was a calm day and the bird was stationary.
500mm lens with 2.0 tele-extender on a Canon 7D. That's an effective focal length of 1600mm! Tripod mounted, locked down, 1/320, f/13. Manually focused using live view, radio remote shutter release.