One who jerks a bird watcher (8)

I believe the answer is:

twitcher

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'bird watcher' is the definition.
(I've seen this before)

I cannot understand how the rest of the clue works.

(Other definitions for twitcher that I've seen before include "Nervous person" , "Rare bird spotter" , "Name for a keen bird-watcher" , "pursuing hobby?" , "Dedicated birdwatcher" .)

I've seen this clue in the Sydney Morning Herald.

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