What equipment do I need ?
The equipment that you will need for bird photography ultimately depends on how good you want your photographs to be, how much equipment you are prepared to carry around with you and last but by no means least, how much money are you prepared to spend!!
The main issue with bird photography (unless the birds are captive) is getting the bird big enough in the frame so that large crops are not required. There are two ways of achieving this. You get close to the subject using hides, camouflage clothing, etc, or you go for very long lenses. In reality, bird photography involves both of these methods and it really boils down to how far you are prepared to go with each facet.
Getting close to your subject can be a relatively easy thing to accomplish to begin with. If you are photographing garden birds (see my article ‘Where to start’) then you may be able to use a garden shed or a conservatory (with some simple method of concealing yourself) as a hide. Place your feeders and perches close by and hey presto, problem number one is solved. The closer you can get the requirement for very long telephoto gets less. Of course, at some point you will want to photograph birds in other places than your garden. The principles remain the same but naturally it becomes more difficult to get really close to your subject and the requirement for longer lenses becomes much more apparent.
So, here is a guide to what equipment to consider. I use Canon equipment so may use this brand as examples.
For the newcomer / or those on a very tight budget
If you are a birdwatcher and have a decent spotting scope, you could try digiscoping. This method involves using your scope as your lens and attaching a compact camera to capture the images. The spotting scope will certainly give you plenty of ‘pulling’ power and the only extra equipment you need with you is a compact camera. The big disadvantage of this method is that it is quite slow. By the time you have focussed the scope, got the camera in position and waited for a response to depressing the shutter, the bird is probably long gone!
Entry Level Digital SLR and low cost zoom lens
An entry level digital SLR camera like a Canon 1000D, 500D or equivalent and a 75-300mm zoom lens could cost as little as £400.
Even ‘entry’ level cameras are fairly sophisticated and are capable of producing decent results. The weak link in this sort of set up will be the lens. The problems with cheap zoom lenses are the optical quality and the fact that they are ‘soft’ when used wide open. To get the most out of these lenses and the sharpest images that the equipment is capable of you need to use an aperture of f8 to f11. This will cause problems because unless there is very good light, your shutter speed will be too slow. To increase shutter speed you will have to increase ISO and this will induce noise into your images. At f8 or f11 it becomes difficult to get the background out of focus. You will also realise that 300mm is not long enough!
If you already have this sort of setup and wish to get better images, invest in a better lens. Consider a Sigma 150-500mm. Its longer focal length will fill the frame much better, though you may still have to stop down to improve sharpness.
Mid priced SLR and ‘better’ lens for the enthuisast.
So, you wish to improve the quality of your images and get yourself more ‘keepers’. As I said earlier, the entry level camera bodies can produce excellent results but they can be lacking certain qualities that you may become aware of. Focusing speed and continuous shooting speed will be better by stepping up to a mid priced body such as the Canon xxD bodies. These also give better handling and speedier exposure compensation adjustment and other useful controls.
Lenses make a big difference to image quality but unfortunately the prices start to increase exponentially. There are a few lenses which could be considered. The Canon 300mm f4 L i.s. , the 100-400mm f4.5-5.6 L i.s. and the 400mm L f5.6. The 300mm is really too short for bird photography but with the addition of a 1.4 extender you get a 420mm f5.6 lens with image stabilisation. The 100-400mm zoom is a very good lens but you will find yourself always using it at 400mm. Optically its not as good as a prime lens. The prime 400mm f5.6 lens is very sharp and is highly respected in the Canon range. OK it hasn’t got image stabilisation but if it did it would cost twice as much. When coupled with a 50D or 60D body it has an effective focal length of 640mm and is sharp even at widest aperture. Overall, I would advise this lens as it scores highly on all points (perhaps with the exception of its lack of i.s.). Current price of the lens is just over £1000.
Serious amateur / pro equipment
If you get to the stage where you find the previous category of equipment doesn’t fulfill your requirements then you need to have deep pockets!
A major step up from the  Canon xxD bodies is the Canon 7D. This bridges the gulf that existed between the xxD and the 1D bodies. (The 5D full frame body is not really aimed at the wildlife,sports market). Much improved focusing, especially with moving targets and better noise control plus many other improvements over the xxD bodies make this a serious bit of kit. If you want to go one better than this then the only option is the 1Dx or perhaps a used 1D mk iv.
As I have stated earlier, the requirement for long telephoto lenses will become apparent all too easily. But now, you are the area of super telephoto lenses with super telephoto prices. The 500mm f4 is, 600mm f4 is and 800mm f5.6 is lenses are superb but they do have their downsides! There isn’t much point in owning one of these lenses if you find it too difficult or too much trouble to carry it around with you. Bear in mind that when you add up the weight of one of these lenses, your camera body,tripod,tripod head and a few other bits of kit you can easily have 25 pounds and more to carry around. Some may argue, but I think that the 500mm is a good compromise. Its heavy, but manageable and with a 1.4 extender becomes a 700mm f5.6. Image quality is superb with all of these lenses, and so it should be with prices ranging from £5,200 to £9000 !
Of course, Nikon sell a similar range of camera bodies and lenses and Sigma sell lenses for most of the camera body manufacturers.