Italian investigation bureau Agenzia Nazionale per la Sicurezza del Volo (ANSV) says the wing surfaces and engines were struck by a "large flock" of starlings.
ANSV has retrieved the flight recorders from the aircraft - which suffered damage to its left wing, undercarriage and fuselage - and is to interview the crew.
The aircraft, which had been approaching Ciampino's runway 15, came to rest on the threshold at the far end of the 2,200m (7,220ft) strip.
Ryanair says there were only a few minor injuries among the 172 occupants. Italian civil aviation administration ENAC expects the jet to be removed from the runway at 02:00 tomorrow.