Having used aperture priority for years in my landscape photography, I have recently starting using manual mode for bird photography.
I have found that using aperture priority does not always give me the control, or the results, I want when photographing wildlife. It is too easy for the exposure to be thrown out when there is contrast between the subject and the background. Additionally, I almost always shoot at maximum aperture (f8 with my set up) to maximise the amount of light reaching the sensor, therefore shutter speed and ISO become the more important variables.
There are three main control variables - aperture, ISO and shutter speed.
Aperture
.....for all practical purposes, in most wildlife shooting situations my aperture is fixed at f8.......
The widest aperture I can achieve when shooting relatively distant wildlife is f8. My Sony 100-400 lens ranges from f4.5 to f5.6 at maximum reach but, when combined with my 1.4 teleconverter, aperture narrows to f8. While I would often like more light than this permits, it is not possible with this set up. On the other hand, I very rarely need to increase my depth of field beyond f8. Therefore, for all practical purposes, in most wildlife shooting situations my aperture is fixed at f8.
ISO
Modern image sensors and post-processing software mean that ISO can be pushed to higher levels than previously would have been possible.
The ISO setting determines the sensitivity of the image sensor to light. The lower the ISO setting, the higher the image quality. Typically landscape photographers use ISO100 and lower shutter speeds for their tripod-based shooting. With wildlife, however, the subject is usually moving and light is not always ideal, hence a higher ISO setting is required in order to achieve a fast enough shutter speed. If the light is relatively constant, it is quite possible to set ISO at a fixed value for the session, at a level appropriate to the conditions. In bright, sunny conditions, this may be ISO400 - in darker conditions it may be ISO1500. It is a question of experimenting with your own gear and shooting situations. I have found that the Sony A7Riv becomes quite noisy above ISO 1500.
Shutter speed
Once set, you can be sure that your subject will be correctly exposed, regardless of the background.
Once we have set aperture and ISO, shutter speed is the remaining variable - the balancing figure. You can test whether your ISO setting is appropriate by framing the subject (or a tree or other object with similar colour characteristics to your expected subject) and looking at the resulting histogram in your viewfinder. If the exposure needs fine tuning (eg. moving the histogram further to the left to avoid clipping of highlights), simply increase your shutter speed until the histogram shows correct exposure - ie. exposed as far to the right as possible but without any clipping of highlights.
Of course, you need to have a shutter speed fast enough to eliminate motion blur. If it is not, then simply increase ISO until you achieve a fast enough shutter speed. Modern image sensors and post-processing software mean that ISO can be pushed to higher levels than previously would have been possible.
I have programmed shutter speed to the large control wheel on the back of my camera, making it very easy to adjust shutter speed on the fly while looking at the subject through the viewfinder.
Once set, you can be sure that your subject will be correctly exposed, regardless of the background.
Caveats
I acknowledge that the above technique is best suited to situations where the light is reasonably constant. If you are photographing in extremes of light and shade, it may be easier to revert to aperture priority to avoid missing the shot while fiddling with ISO and shutter speed settings. I suspect, however, that if you persevere long enough, using manual settings will become second nature and suitable for most conditions.
An alternative would be shoot in shutter speed priority and use auto ISO.
This advice is aimed at users of mirrorless cameras, where a live histogram is available in the viewfinder. The principle is similar for DSLRs, but you will need to take a test shot first, and then examine the resulting histogram.
Give it a go
I recommend giving this technique a go, as a way of improving the consistency of your results. Even if you decide it is not for you, it may at least give you a greater awareness of your camera settings and the inter-relationship between aperture, IOS and shutter speed. I am going to keep using it for the next few weeks and will see how I get on.