Why I LOVE Manual Focus
I used to be terrified of focusing manually. On most auto focus lenses, the focus ring is small and more difficult to use... also there's something really easy about just allowing your camera to do the focusing work for you. I was afraid I couldn't react quickly enough to moving subjects and that I wouldn't be as good as my camera's auto focus. Now I see the errors of my ways.
For wildlife (or people), you want to make sure the main subject's eyes are in perfect focus. You won't be able to sell any image if the eyes aren't in focus. If you shoot a subject 10 feet away at 2.8 and use auto focus, the camera will choose the object closer to the camera (usually the nose, cheek, or eye brow... not the eye itself). A 2.8 aperture means that you will have such a soft depth of field that the eyes will appear out of focus. The older (and more affordable) manual focus lenses have the most beautiful focusing rings you've ever seen. I find it much easier to use manual focus on my 300mm f4 lens from the 1970s than my newer autofocus 80-200mm zoom (using the auto focus feature). Unfortuately, I find the focusing ring on the newer models a little bit pooey... but I did want to make the case for why I think manual focus lenses from the 1970s are the greatest things since sliced bread!
Rule of Thumb for Fast Subjects
You may already be aware that your shutter speed should be at least the same as the distance of your lens. For instance, you need to shoot at least 1/300th of a second if you are using a 300mm lens or hand shake will make your picture look really blurry. And anything larger than 300mm should be put on a tripod (preferably one with a ball head for wildlife work). With magnification, you may be able to get away with shooting a 300mm (a 450mm on your digital) hand-held... at 1/450th of a second or higher...)in a pinch... but investing in a good ball head tripod will really improve your results if your lens is higher than 300mm.
Back on the subject of pairing your shutter speed with lens distance... if you are currently shooting P or Auto it's a good idea to change to shutter priority instead. If you're shooting with a 200mm make sure your shutter is set at 1/300 because of the magnification. All modern digital cameras will then set the aperture to pair with the shutter speed. At the same time, practicing photography on manual settings is always a good idea because it will offer you greater control.
I could write for days about shooting nature and wildlife photography with Nikon equipment but I'll stop for now. You're welcome to read more of my nature photography articles online by clicking at the link to "wildlife photography articles" at the top of this page.