Snapped quite a few snaps of some birds whilist visiting a costal town south of Oslo on the weekend. If you look at the picture, you'll notice that there is a lot cloud. In fact, there wasn't a bit of blue sky anywhere. What there was however, was plenty of snow, and plenty of water.
In front of me was the above image, and behind me plenty of more snow. There was an opening expanse of water image right and a river image left. In short, a bit of a nightmare for automatic camera operations. If you were to metre light for the clouds and then move over the snow, you're settings would change somewhat. Add the fact that snow often throws of the metering regardless due to the excess white, automatic modes such as TV and AV were out of the question.
If you look at the above image, the sky, snow and water are correctly exposed. Doing that with AV or TV would be difficult without exposure compensation. This can make it tricky to keep up with your settings when shooting fast moving things such as wildlife.
Light not changing?
Use manual. If the light isn't chaning, switch to manual and dial in your exposure. Once you've got it, that's it. Shoot and forget. Ok, so if the light starts to change or you've been at it a while, best to double check, but you won't have to change it that often.