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.
Never underestimate the quality of light necessary for a good photo. I prefer to work in rarer lighting, either early or later in the day, under overcast skies or the light at the edge of a storm. These light sources add more drama to your picture. Cloudy bright days also more nearly match the contrast range of your film. Thus, you are not loosing detail in either the highlights or the shadow area of your photograph.