Critters are most active in the early morning or late evening. It's as if they have no respect for photographers who want to photograph them. Seriously, the nerve of these animals.... Anyway, in order to photograph them when they are active you have to shoot in some pretty low light situations. Here you will find some suggestions on how to make the best of what nature provides you.
Aperture. The Aperture is the opening in the lens and you want as large as you can get as to let in as much light as possible (smaller f-number). You will want to be as wide open as is reasonably sharp for the lens you have. No all lenses are sharp at the widest aperture so you will need to do some homework to find out what's best for your lens.
Shutter Speed. The shutter speed typically needs to be 1 over the focal length of the lens to get a good clear shot devoid of camera shake. So if you have a 300mm lens you need a shutter speed of at least 1/300th of a second. With many lenses now having Vibration Reduction you can go a bit slower. In fact, if you are photographing animals in dim light you may have to be much slower. This may be fine if you are on a tripod and the critter isn't moving fast.
ISO. ISO represents your camera's sensitivity to light. The higher the ISO the better you can shoot in lower light. There is a trade off as there always is in photography. The trade off is noise. As you increase the ISO you also increase the noise but this may be your only option in certain situations. It's better to have some noise than to not get the shot at all. Most modern cameras have good ISO performance.
Lens Choice. Typically you need a large and fast lens for wildlife photography. You need to be able to get close to the animal without physically getting close, hence the large focal length lens. Unless you enjoy challenging a grizzly for its territory and if you do then perhaps you should be reading a psychological disorder page rather than this one.
You will also need a fast lens to let in as much light as possible. So, what does a large fast lens equal? A new mortgage on your house. Most of us can't really afford a 400 f2.8 or a 600 f4 so we go for a variable aperture zoom lens such as the Nikon 80-400 f4.5-5.6. These let in less light so we have to consider Shutter Speed and ISO to help make up the difference.
Burst Mode. Burst Mode not only sounds cool but it will allow you to get sharper images. What? Yeah, you heard me... Sharper images. When you lay down on the burst mode you are firing off tons of shots. There is a good chance one or more of those shots will be at the exact right time giving you a sharp image. Plus, this increases your chance to get the subject doing something interesting.
Tripod. Seriously... You need a tripod. Not a monopod and not hand held. We are talking about you being out before the crack of dawn shooting a moving subject with a large focal length lens. A good and stable tripod will help keep camera shake out of the equation. This will allow you to lower your shutter speed giving you some additional light. If the subject isn't dancing a jig you should be golden.
So, what advice do you have when photographing wildlife in low light?