I can remember when we took off from San Diego in the spring and there was an enormous amount of seagulls right along the shoreline where I was making my turn to land at the airport. I knew almost immediately that had I continued my course that I would probably go through the flock, so I quickly radioed the ATC's that I was climbing NOW! We went into a steep climb to avoid the birds while also scaring some of the passengers. After we had leveled off, we had to do a go around, so I had the F/O make an announcement letting the passengers know that we had to take evasive action, rather than risk ingesting any birds and maybe having to make an emergency landing. As the passengers exited the plane after landing, some of them thanked us for being alert and quick thinking. I felt very good about the decision we made and we also did not have to make an NTSB report.