Even though modern DSLRs are able to meter through the lens, that doesn't guarantee your subject will be exposed appropriately. Most cameras have several different metering modes: an evaluative multi-segment mode, a center-weighted mode, and spot meter mode. For stationary subjects spot metering with AF-S almost always will give the best results. What must be prevented is over-exposing any part of your subject.

Regions that are under-exposed can be recovered during post-processing (PP), but over-exposed regions cannot. In addition, if some areas are over-exposed, then other bright areas will have reduced contrast even if they are not over-exposed. Check your camera settings when using AF-S to see if exposure can be linked to focus point. If so, then aim at the brightest part of your subject, press the shutter halfway to set focus and exposure, and then recompose before pressing the shutter fully. If you can't link exposure to focus point, then you may need to adjust automatic exposure compensation to shoot the scene darker than what the meter ordinarily would choose for exposure. This also may be needed when shooting BIFs with AF-C, especially if you are shooting white birds.

White birds such as Egrets, Swans, and White Pelicans are a challenge to expose properly, especially in bright sunshine. I make sure to under-expose these subjects. Not only does that ensure I don't over-expose any of those bright white feathers, it also gives me headroom to stretch the contrast of white areas during PP, making them more interesting instead of just uniformly bright white.

When shooting stationary subjects with AF-S, you normally want to control DOF, so using aperture-priority mode is appropriate. This lets you control f-ratio, while the camera's meter selects a shutter speed that is appropriate for the available light. In this case, you need to monitor shutter speed. If it approaches a speed that is too slow for the conditions, then you should increase ISO.

Snowy Egret at the spillway steps
Pentax K5 500mm f/7.1 1/1000 ISO 500

When shooting with AF-C, you usually want to control shutter speed, so you should use shutter-priority mode. The camera's meter will adjust lens aperture for the available light. If the selected aperture approaches the lens minimum, then you should increase ISO to keep lens aperture within your camera's range of sweetness. When shooting fast-moving birds like ducks, try to shoot at 1/2500 sec. or faster so you won't get any motion blur. Slower moving birds like white pelicans or soaring hawks can be shot somewhat slower, but they still need 1/1600 or faster if possible.

My current camera, a Pentax K3, along with most other Pentax DSLRs, has a third mode called TAv. This mode lets me set both shutter and aperture while the camera's meter adjusts ISO to the appropriate level. I monitor ISO in the viewfinder to see if it is getting close to its minimum ISO of 100. If that happens then I know the camera is close to over-exposing, so I need to use a faster shutter or stop down some. If the selected ISO gets too high, then I need to open up the lens or use a slower shutter speed. This is the only mode I use for wildlife photography.

Here is an example of the application of these focus and exposure principles.

Several years ago we had a fairly large February snowstorm. I went to White Rock Lake to look for photo opportunities and found two White Pelicans huddled together in the middle of a small cove, enduring what they thought they had left behind by coming down here from the northern plains. I wanted to have most of their reflection be in focus and I did not want to blur out the background, so I set the camera lens to f/8 to get higher DOF. Originally I set the shutter to 1/500 sec., but when I metered on those white feathers, ISO went down almost to 100. That told me I was close to over-exposing the feathers, so I changed shutter speed to 1/750. Here is the result:

White Pelicans on a snowy morning
Pentax K20D 300mm f/8 1/750 ISO 280

This image also shows that another way to isolate your subject from its surroundings is by brightness contrast. Since I wanted the background to be recognizable, I was able to isolate these pelicans by the contrast between their bright white feathers and the dark water. This also required that I shoot from an angle that left some dark water between the pelicans and the snow-covered shore behind them. I heightened this isolation by increasing contrast of the image during PP, something that would not have been effective if I hadn't left some headroom by under-exposing the pelicans. These are examples of things one must consider in order to move from taking snapshots to taking photographs.