REMEMBER: Tips are the bare minimum amount of knowledge for a particular task. It is not intended to include anything other than a starting point for your photographs. In most cases, this will be enough to get you started.
A full discussion for all tips will come with time.
TIP: Ten Ways to Improve Your Nature Photography
1. Get closer - whether by physically getting closer to your target, by the use of longer lenses, or both.
2. Use a tripod or a monopod.
3. Slowly release the shutter.
4. Learn about your subject so you can anticipate opportunities for the best shots.
5. Compose for the best image in your viewfinder.
6. Use the rule of thirds for composition.
7. Be aware of light and learn how to use it.
8. Understand how, when, and why to adjust your shutter speed, aperture, and ISO.
9. Get your camera off Automatic or Program.
10. Learn how to use your histogram.
For more information, check out Animals Just Won't Say "Cheese" - Taking Better Nature Photos.
What is more important the aperture or the ISO?
It all depends upon the situation.
The aperture controls the depth of field. Shooting birds in flight usually takes place at a distance and the depth of field may not be critical as long as you catch the eyes and body in focus. Most of the time, that will not be the difficult part of an acceptable shot. You will often have to shoot with your aperture wide open in order to use the high shutter speed that you need.
ISO will be important, of course. You want to use as low an ISO setting as possible to get the best possible image, but you may have to sacrifice a bit of quality and endure a bit of noise in your photos to be able to use the high shutter speed you need to stop the movement and get a sharp image of birds in flight.