I have been photographing birds for only a couple years, although I have been taking photos longer. These are some things I learned by trial and error.
1. Try to photograph the bird either close up, or with enough magnification so that the bird fills a good part of the frame.
2. Lighting is very, very important. Either try to photograph the bird with good natural light, or learn to use an outdoor flash. Good natural light generally is light in the early morning or in the late afternoon, with the sun either at your back or to the side of the bird. One of my next goals is to learn outdoor flash photography.
3. Pay attention to composition. Learn the rule of thirds, but use it only as a general guide.
4. Aim the active focus point on the eye. If the eye is not in focus, the photo will not be nearly as good.
5. Decide how much depth of field you want. Use a lower f stop or higher magnification lens, or both, for more shallow depth of field. If the background is busy, you may want to use shallower depth of field.
6. Experiment with use of the Av, Tv or Manual modes and with evaluative or spot metering. In general, for still birds I like to use the Av mode and the evaluative metering mode (except I sometimes like to use the spot metering mode if there is high contract between the subject and background). For birds in flight, I like to use the Tv mode for hummingbirds (so I can control freezing the wings) and either the Av or M modes for larger birds in flight. These are just my personal preferences. I am by no means an expert.
7. Use exposure compensation liberally. I will set the exposure compensation to what I think will work best, and then review both the histogram and the photo in the LCD display. I will then adjust the exposure compensation if I think it is necessary, again reviewing the histogram and photo in the LCD display. Read about histograms, and routinely watch the histogram.
8. Use the lowest ISO setting that will still result in enough light to get a good exposure. Turn up the ISO as necessary.
9. Watch for interesting bird poses, such as the head at an angle but the eye still visible, and interesting settings and backgrounds.
10. Photographing birds in flight is a different skill. Use a continuous focus mode. Learn to lock the focus on the bird, and then to track it while depressing the focus button half way. Learn whether you need to use the center focus point on your camera or whether other focus points will work okay. Use a longer focal length lens (one with higher magnification). Pay attention to which custom function settings will work best.
I attached a few photos to give examples.
Photo 1.
In the late afternoon, I set up my camera on a tripod about 20 feet from a water feature in my backyard where small birds like to land. I used a 500mm lens, so the bird would sufficiently fill the frame. I stood still, and waited and waited.
Note the the sun in this photo is to the left of the bird, casting a shadow to the right. The rocks in the top corner and bottom right are perpendicular. The eye of the bird is in focus. The photo shows detail in the feathering of the bird. I like this photo, but it lacks some interest because of the pose of the bird.
Photo 2.
I like this photo better than the first one. The bird's head is turned toward the camara at around a 45 degree angle, which I thinks adds a lot of interest. The eye is in focus, and the feathering detail is shown. The rock in the upper right and lower left are depicted at a different angle than in the first photo, and the bird is facing in the other direction making a V between the bird and the rock. I like the composition better.
Photo 3.
This a photo taken in the late afternoon with the sun at my back. The background is blurred, and the juxtaposition of the bird, the blurred tree in the background and the sky adds some interest. But there is really nothing very compelling about this photo. The bird is a little blurry, and is not in a very interesting pose.
Photo 4.
I think this is a much better photo than photo 3. The bird is in a much more interesting pose, where the head is turned but both the head and front of the bird are well depicted. The eye is in good focus, and the feathering detail is shown well. The background is busy, but is sufficiently blurred so it is not distracting.
Photo 5.
This is a small bird in flight, coming in for a landing at a bird feeder. This type of photo takes a lot of practice, and at least for me requires some luck and taking a lot of photos (mostly bad). Photographing small birds in flight is much harder than photographing larger birds in flight, because small birds move so much more quickly and there is less bird at which to aim the focus point. This is one of my best efforts.