Taking acceptable pictures of wild birds in natural settings requires a great deal of devotion and patience, and some knowledge of the natural history of the species with which you are working, if you want anything other than random snapshots of whatever species you might be lucky enough to approach. Because birds are often nearly constantly in motion, any one shot is likely to be unsuitable due to blur or an unappealing posture, turned head, or closed eye at the moment you choose to open the shutter. Consequently, most bird photographers burn lots of film, and expect a low proportion of truly excellent photographs. Here are a few of my recommendations based on more than 20 years of taking mostly mediocre to poor bird photographs. For a much more detailed tutorial on bird photography, see Bill Horn's excellent instructional web page .
Longer lenses are better.
I use a 400 mm lens, which is usually considered the minimum acceptable focal length for serious bird photography. With a 400 mm lens, you must be about 10' away from a bird the size of a cardinal to get a frame-filling image. You are very, very rarely 10' away from a wild bird, and when you are, it's usually not for very long. Even for larger birds such as herons and egrets, you still have to be quite close to the bird to get a frame-filling image. Really devoted bird photographers typically opt for 600-800 mm lenses, often used with teleconverters, and still complain about not having enough focal length most of the time.