Learn How To Photograph at High ISO
Understanding the need for fast shutter speeds, you now have to deal with the disadvantage of allowing less light to get to the camera’s sensor. In order to compensate for that, you are going to have to increase the ISO. But, a higher ISO value can lead to grain in your images.
If the light is really bad, you’ll have to select a high ISO value. The good thing about modern full-frame cameras is that they are able to shoot at a higher ISO value (up to 6400), without having much grain or noise in the images.
ETTR (Expose To The Right) is a technique where-by your slightly overexpose your image above an optimal setting. The basis behind it is that you get the most noise and loss of detail in the dark areas of a scene. When overexposing the image slightly you will get that information in the shadows where you would otherwise see noise.
Instead of boring you with the mathematical explanation of ETTR let me simplify the details. Think of a histogram divided into 4 equal parts, from right to left: highlights, light mid-tones, dark mid-tones, darks. Here, ½ of our tonal values come from the highlights, ¼ from the light mid-tones, 1/8 from the dark mid-tones, and 1/16 from the darks. 50% of the detail in your image is therefore in the highlights if you don't overexpose your image.
If you overexpose your image (without clipping the highlights) you will be recording more tonal information, which also leads to less noise on your image. Of course, the resulting image will be somewhat washed out. But you have recorded more data, which makes it easy to darken the image back to the proper exposure in post-processing without adding noise.
When it comes to wildlife photography, experiment with ISO settings to figure out your camera’s limits. Keep in mind that good image quality is quite important for the overall impression of your photograph. If there is a lot of noise in your image, you can always reduce it while post-processing your image.