Birds are unique in that they are often relatively small and often are quite a distance from the camera. That seems to say that a very long lens is necessary for successful birding.
However, what does one do when a focal length longer than 300mm (300mm seems to be the limit of affordable lenses in both the Nikon and Canon lines) is not possible to purchase?
The answer is simple but just because it is simple doesn't mean that it is easy to do: GET CLOSER TO THE BIRDS!
Just how does one get closer to these nervous creatures without scaring them into flight... One answer might be; let the birds come to you. Using some sort of a blind (it is often called a hide by people who speak British English). This can be quite an intricate thing or it can be somewhat simple. One of the most simple types of blinds might be a folding chair with a hood.
http://www.ebay.com/itm/Quik-Shade-F...gAAOxyqjBTl27z
Clip some gardening shade cloth or camouflage mesh, long enough to hit the ground, to the awning with a hole cut out or a slit cut for your lens.
Placing that chair (which will now hide your outline and even hide slight movements) in an area where you expect the birds to congregate. Using some bait to attract these birds is not frowned upon like it is in sport hunting. Also using a bird-call recording ca be quite handy. I got very competent in using a wooden duck call. I have been also able to attract elk in the rut when I lived in New Mexico. Unfortunately that was before I had a digital camera.
Here is a YouTube video about using a blind for birds/
https://www.youtube.com/watch?v=wbmWt-FSHSs
Tony Northrup uses a hunting blind - however the chair and mesh should do as well.
Single person tent-blinds can be available for a lesser price. But the nice thing about the chair blind is that you can use it for other than hiding from birds...
There is another trick for getting photos of birds. Instead of using a DSLR with a long lens, opt for a bridge camera. I have a Canon SX-50 HS that I purchased refurbished for less than any new DSLR and long lens. The SX-50 HS has a built-in lens with an equivalent maximum focal length of 1,200mm. Their are other bridge cameras that have as long of an equivalent focal range but which have a faster aperture. The SX-50 HS does achieve some fairly decent imagery especially when mounted on a tripod.