Twitching is a British term, meaning “the observation of a previously located rare bird”. In North America, this is often called chasing.
The goal of twitching is often to accumulate species on one’s lists. Some birders engage in competition with one another to accumulate the longest species list. The act of the pursuit itself is referred to as a “twitch” or a “chase”. A rare bird that stays put long enough for people to see it is called “twitchable” or “chaseable”.
Twitching is highly developed among birders in the United Kingdom, the Netherlands, Denmark, Ireland, Finland and Sweden. The smaller regional size of these countries make it possible to quickly travel inside their borders with relative ease. The most popular twitches in the UK have drawn large crowds, such as a group of approximately 5,000 people who came to view a Golden-winged Warbler in Kent.
In the United Kingdom, twitchers have developed their own vocabulary. For example, a twitcher who fails to see a rare bird has dipped out; if other twitchers do see the bird, he may feel gripped off. Suppression is the act of concealing news of a rare bird from other twitchers. Similar vocabularies have developed in other countries where twitching is popular.