Some nice shots. Here are my comments;
1. Like DQ said, "where the legs". Always try to take as much of the animal as you can (unless you specifically want to take a head and shoulders portrait), and leave some space for the animal to move into the picture (Rule of thirds will also help in this regard). Remember you can always crop later on the PC but you can't un-crop something taken on the camera.
2. The Giraffe shot is lovely, good lighting. Best times to take photos are early morning and late afternoon, when the sun is lower on the horizon (Funnily this is also the best time to see game, strange how these things just work out). Also don't be scared to take a vertical shot. In this case it would have helped to fill the frame and add more to the story.
3. The Lion, I love the tongue. Everyone has seen a lion, but how many licking his lips. Getting the right shot at the right moment to add more to your photograph. Improvements, depends on what you want? If you want everything frozen in time, including the grass blades, then use your shutter priority and a fast lens (f/2.8, but let the camera worry about that and you worry about the shutter speed). If you want the lion crisp, and the grass blurred, use aperture priority (around f/4 to f/5.6, depending on how strong you are). You want to pan with the lion. This should hopefully blur the grass and give you a decent image of movement from the lion. You may want to consider an external flash to help with the lighting.
4. As with point 2, try to get this shot at a different time. The high mid day sun tends to wash out nature's wonderful colors. If you have no choice, try to increase the contrast later on the PC. You may also want to look at using exposure compensation to darken the image a bit. The foreground objects (blurred branches and leaves) detract a bit from the photo, especially where they cover the buff. Try zooming in a bit or even moving (if possible, wildlife is not the most accommodating model).
5. Not much to say. Good photo. If you want it might help to switch to manual focus so you can prevent the camera from getting confused and focusing on the branches instead of the bird.
That's it for now. My last tip, lots of practice. Especially if you want to start using aperture priority. Don't get caught up on trying to keep the aperture as open as possible (i.e. f/2.
. Try all the different apertures on a stationary target first and try this at different time of the day. You'll fail a few times but as long as you learn from the failures you'll be fine.