Twitchers are the extremists among birders. According to Wikipedia: "Twitching is a British term used to mean "the pursuit of a previously located rare bird." In North America it is more often called chasing, though the British usage is starting to catch on there, especially among younger birders. The term twitcher, sometimes misapplied as a synonym for birder, is reserved for those who travel long distances to see a rare bird that would then be ticked, or counted on a list."

Most of us prefer birder or birdwatcher. :)