A Twitcher is a
birder who is willing to go somewhere purely to see a
bird which is new for their
list. What an individual birder considers to be a twitch can vary. For some any journey, no matter how short, constitutes a twitch if its primary purpose is to see new
species.
The extreme form of twitcher is one for whom the list is more important than the bird. This makes birding little different from trainspotting. As a result, many birders consider twitcher a derogatory term for their hobby.
Twitchers tend to be pro splitting as splitting has the effect of increasing their list without even having to leave the house.