The Ryanair plane that made an emergency landing at Ciampino Monday was hit by a large flock of starlings, Italy's flight safety agency ANSV said.
ASNV have removed the plane's black box and are analysing data.
They said the plane had ''extensive'' damage to its left wing, left undercarriage and belly of the fuselage.
Civil aviation authority ENAC said the plane, flying in from Frankfurt at 08:00 Italian time (06:00 GMT), veered onto the side of the runway and skidded to a halt on its side after ''problems'' with one of its landing wheels.
Passengers were unhurt apart from five people with bruises, ENAC said.
''We were landing when we suddenly smelled something burning,'' said Gaetano De Caro, a Sicilian who lives in Germany.
''We hit the runway hard and we were really scared but the pilot was great...he saved our lives.
''If it had been someone else, perhaps, we would all be dead because we were getting close to the perimeter wall,'' De Caro said.
ENAC said Ciampino would be operational at ''about'' 17:00 Italian time (15:00 GMT), when the plane would be removed.
The authority said it would name its own expert to look into the incident.
There have been some 600 collisions with birds in Italian airports over the last year.
The Italian Air Force has lost three fighters because of bird strikes in the last ten years.
