I believe the best bird photography is obtained by photographing birds in their natural surrounding in the wild. When birds are photographed in aviaries they generally do not look natural in their surroundings.
To photograph birds naturally requires lots of patience and long lenses. Many varieties of water birds are quite large and can be captured with lenses of around 300mm. But many smaller birds such as wrens and robins will require longer lenses of up to 500-600mm.
There are several methods that can be used to get closer to wild birds. Most birds are very timid and will fly away quickly as you approach. One successful method is to stalk the bird by slowly moving toward the bird stopping for a few seconds, then gradually moving forward again and stopping, if this is done slowly you can sometimes approach quite close to the bird. Using this method it is also possible to get a flight shot of the bird as it takes off.
Another successful method to get close to birds is to use a bird hide, the hide is placed close to where birds frequent so that they get used to it and then you wait in the hide for the bird to approach. This method can be quite successful if the hide is placed close to where the bird may come for water or food.
Some of the best bird photography I have seen is in the links in this blog.