This is a lot more common than I thought. Who knew that commercial aircrafts were so vulnerable? From Live Science:
More than 200 people have been killed worldwide as a result of wildlife strikes with aircraft since 1988, according to Bird Strike Committee USA, and more than 5,000 bird strikes were reported by the U.S. Air Force in 2007. Bird strikes, or the collision of an aircraft with an airborne bird, tend to happen when aircraft are close to the ground, which means just before landing or after take-off, when jet engines are turning at top speeds.
The incidents are serious particularly when the birds, usually gulls, raptors and geese, are sucked into a jet engine and strike an engine fan blade. That impact displaces the blade such that it strikes another blade and a cascade can occur, resulting in engine failure.
A 12-pound Canada goose striking an aircraft going 150 mph at lift-off generates the force of a 1,000-pound weight dropped from a height of 10 feet, according to Bird Strike Committee USA.[...] Large aircraft are certified to be able to keep flying after impacting a 4-pound bird, however 36 species of birds in North America weigh more than this, according to the committee. Even smaller birds, such as starlings (which the committee's Web site refers to as "feathered bullets" due to their density), can cause engine failure.
The greater the difference in the speed of the plane and the bird, the greater the force of the impact on the aircraft. The weight of the bird is also a factor, but the speed difference is a much bigger factor.