A tripod is also a very essential item to obtaining sharp images. Always buy a quality one. I have found a window mount to help while photographing from my car. The car serves as a blind and the bird will let you approach much closer. In Ding Darling, I have gotten to within 3' of a hawk.
Some people use bean bean bags or bracing arm devices on their tripods.
Canon has been making image stabilization lenses for a while but you will need to save your pennies for this one. The most necessary item for successful bird images is patience and it can't be bought at a camera store. Remember just because you finally arrive in bird paradise they may not want to cooperate. It's not always easy to get great photos. Good light is a major determinate to a great photo.