The HH-60 Pave Hawk helicopter, operating out of RAF Lakenheath in Suffolk, crashed near Cley next the Sea on January 7 during a routine training exercise.
The US Air Force (Usaf) issued the findings of its investigation yesterday, saying Brigadier General Jon Norman, president of the inquiry's board, had "found clear and convincing evidence that multiple bird strikes caused the mishap by rendering the pilot and co-pilot unconscious and disabling the trim and flight path stabilisation system".
Captains Christopher S Stover and Sean M Ruane, technical sergeant Dale E Mathews and staff sergeant Afton M Ponce all died when the helicopter came down on marshland near the village at around 6.05pm.
All were serving with 56th Rescue Squadron, 48th Fighter Wing, which carries out search-and-rescue operations
The report, compiled by the Accident Investigation Board, said the purpose of the training mission was to recreate a night-time rescue scenario involving a downed F-16 pilot.
It added that as well as the loss of life, the crash cost the US government an estimated at $40 million (£23.5m).
It read: "A flock of geese took flight from Cley Marshes, likely startled by the noise of the approaching helicopters, and struck the MA (mishap aircraft)."
Richard Kelham, chairman of Cley Parish Council, said residents had long-held concerns about low-flying helicopters over the marsh, which is a Norfolk Wildlife Trust bird reserve.
He said: "It is inherently dangerous to fly low over an area with a lot of birds, and hopefully lessons can be learned from this tragedy."