David Campbell, who traveled to Harris from Surrey in the London suburbs after learning of the white-throated needletail's presence, told BBC Scotland he saw its end Wednesday afternoon. About 30 birdwatchers who had made the trip were present.
"We just watched the whole thing with dismay," he said.
The needletail, a relative of the common swift, nests in Siberia and winters in India, Southeast Asia and Australia. Only five sightings have been recorded in the British Isles since 1950.
It is believed to be the fastest bird in level flight with speeds as high as 105 mph.
Rare Bird Alert, which notified birdwatchers of the needletail's presence on Harris in the Outer Hebrides, recorded its death: "The white-throated needletail on Harris flew into a wind turbine and has died, pathetic way for such an amazing bird to die."