Ryan, if you shoot in manual adjusting either the aperture or shutter speed will effect the exposure.
If you point your camera at a subject in manual your meter will give you a reading, by changing the aperture or shutter you can get the meter to show the cameras ideal exposure normally you see a scale in the camera from -3 to +3, when its on zero its what the camera believes is the correct exposure. Each increment is a stop of exposure so +1 is one stop over and -1 is one stop under.
When you shoot a dark subject against a bright background such as a person in front of a window or a bird against the sky, unless you spot meter off the subject the camera will be fooled, in these cases you would normally add exposure by either slowing the shutter or opening the aperture or using exposure compensation. A person in front of a window is easy as as I said you could spot meter off the subject and the camera would ignore the bright background, with a bird in flight its not so easy so photographers use the ability to over expose the image to improve the subject at the expense of the background (depending on the background).
Its normally trial and error, you could try it by pointing the camera at a branch that has the sky as the background, set the camera to say aperture priority and full metering take a shot, then using the exposure compensation change the exposure to +1 and take another, then +2, then +3 look at the 4 images and see the difference, do not change the aperture.
Likewise if you take a picture of a swan against the water and the water relatively dark, you would need to either meter off the swan or under expose or the swan would be over exposed.
If you tell us what camera / lens combo you use it might help contributors to the thread assist in more detail.
I've no doubt the manual with the camera cover exposure compensation in more detail.
The attached image shows what is needed, the bird is relatively dark against a dark water but in strong sunlight to the right the white feathers would be burnt out, I had to shoot this at around 2 stops under exposed to save what I could of the highlights but sacrificing a little shadow detail, sometimes it works well and I like the feel of this shot.
Ahh I've just seen you state a 50D, never mind, you may want to pose the question in the canon section as to how to use exposure compensation on a 50D, being a Nikon man I can't help
Additional thought, your camera will be able to bracket, try a sequence of say 7 shots from -3 to +3, choose a subject and mount the camera, set it to say aperture priority and full metering and then shot the 7 shots, you will see the effect on each shot, you should be able to tell quite easily which one is -3 or +3