Wildlife photography has three basic rules:
1. make sure your subject is properly focused;
2. make sure your subject is properly exposed;
3. be prepared.

Of course, the devil is in the details. All cameras have a sweet range -- settings with which the camera performs best. You need to know where your camera's range of sweetness can be found. Not all conditions will allow you to stay within that range, so it is important to understand what compromises can be made and how to make the right one for the situation.

Focus. Most DSLRs have two autofocus modes: AF-S and AF-C (different cameras may refer to these using different names). AF-S is single shot AF mode. In this mode you can aim at a particular spot and then press the shutter halfway. At this point the camera sets focus and the meter adjusts whichever parameter, aperture, shutter, or ISO, it is able to adjust. If the camera is moved while holding the button halfway, the camera will hold focus and optionally hold meter setting. This lets you recompose while focusing and metering on a critical part of the scene. Focus and metering won't be updated until you release the shutter and press halfway again.

Barred Owl Juvenile: Learning to hunt
Pentax K3 420mm 1/320 f/5.6 ISO 5000

Bird-in-flight (BIF) shots are among the most dramatic and visually interesting wildlife images one can take. However, they also are among the most challenging. BIFs require use of AF-C focus mode. This mode continuously adjusts focus while the shutter button is pressed halfway. Of course, you also should use continuous shooting mode for the shutter so that the shutter will fire as long as you hold the button down. However, it is critically important that you don't press the shutter fully until the subject is in focus. Once the shutter is pressed, the camera assumes whatever is in the focus area is focused the way you want it and then tries to maintain that focus as long as you keep the shutter pressed all the way. So watching your subject in the viewfinder with the shutter half-pressed and waiting until it is in focus before completely pressing the shutter are the keys to successful BIFs, along with practice, practice, practice.

Brown Pelican at Jupiter Inlet
Pentax K3 300mm 1/2500 f/5.6 ISO 200

Depth of Field (DOF) is another component of proper focus. This refers to the distance behind your subject that remains in focus. DOF is controlled by lens aperture, or f-ratio. The higher the f-ratio, the greater DOF will be. Ordinarily, we want to have our subject isolated from its surroundings. One way of doing this is to shoot with a narrow DOF so the background is blurred. That requires using a small f-ratio. Each lens has a minimum f-ratio. A lens at that setting is said to be wide open. The f-ratio represents the ratio of the lens focal length divided by its diameter. A lens can have a greater f-ratio if its diameter is decreased, so lenses have diaphragms that partially close to decrease their effective diameters. This is referred to as stopping down. Most lenses lose some sharpness if they are wide open, especially around the edges, so try shooting one stop higher than the minimum f-ratio of your lens. I mostly shoot wildlife with a Pentax 300 mm f/4 lens and routinely set it at f/4.5 because this lens hasn't lost any sharpness at that f-ratio. This also lets me shoot with a faster shutter speed or lower ISO.

Green Heron hunting
Pentax K5 300mm f/5.6 1/2000 ISO 1600

A common misperception about shooting BIFs is that stopping down to increase DOF will give a better chance of getting the bird in focus. However, doing that will require using a slower shutter speed potentially resulting in motion blur, or it will require higher ISO resulting in noisier, less sharp images. Furthermore, even if your subject is in focus, the background will have less blur and so will be more distracting. Instead, practice, practice, practice using low f-ratios.

One exception to shooting with short DOF occurs when I'm shooting butterflies. Usually those shots are taken at fairly close range, so I stop down to f/6.3-f/8, depending on distance, so that all of their wings and their antennae will be in focus.

Gulf Fritillary after the rain
Pentax K20D 500mm f/6.7 1/180 ISO 140