A flock of geese caused the deaths of four US airmen whose military helicopter crashed over a nature reserve in eastern England, investigators said Wednesday.

The US Air Force HH-60 Pave Hawk, based at Lakenheath airbase, came down in Norfolk on January 7 while on a training mission.

As the helicopters approached, a flock of geese flew off from the reserve in Cley-next-the-Sea, probably startled by the noise, the military investigators' report said.

At least three birds crashed through the windscreen into the cockpit, knocking the pilot, co-pilot and aerial gunner unconscious, said a statement from the Accident Investigation Board, issued by US Air Forces in Europe and Air Forces Africa.

One of the geese also disabled some of the flight controls of the chopper, whose crew were practising a night-time rescue of a downed F-16 pilot at the time of the crash.

"With the mishap pilot and mishap co-pilot unconscious and the trim and flight path stabilization disabled, the MA's (mishap aircraft's) cyclic stick, which controls pitch and roll of the aircraft, was free to move randomly," the statement said.

It added: "The MA impacted the ground... approximately three seconds after being struck by the geese."

The helicopter was from the US Air Force's 56th Rescue Squadron, 48th Fighter Wing, also known as the Statue of Liberty Wing and a key component of US air power in Europe.

Pave Hawks are used for combat search and rescue and often help retrieve downed aircrew in hostile environments.

They have been used in Afghanistan, Iraq and in the aftermath of Hurricane Katrina in 2005 to support recovery operations in New Orleans.