Obviously, a lot of bird interactions are also examples of other behavior, like child rearing or feeding. From a photographic perspective, I think the main thing to keep in mind is that an interaction between two birds is usually the most interesting thing you can capture in any given scene. Always be on the lookout for birds interacting with each other, or ones that look like they are about to. This can be a great way to focus more carefully when photographing a large flock. Also, common birds interacting with each other may be more exciting to photograph than a more unusual bird doing nothing.