Anyway, to answer torquelink's bird ingestion comment. An engine could probably tolerate a goose size strike. But it's doubtful you can make an engine that would survive several geese
Well it depends on the size of the engine vs the size of the bird and precisely where it hits the fan.
If you hit near the center then the fan blades fare pretty well, while a hit near the tip will often mess things up in the fan enough to disable the engine.
As the bird size starts to fill the inlet then the odds of getting a whole bird in without breaking it into small pieces off the inlet lip increase. So multiple whole 8 lb birds into a CFM56 would be improbable. Fortunately mother nature provides clearance between the wing tips of flying birds so the areal density of the really large birds in a startled flock spread out far enough to make it unlikely that you can get more than one whole bird into an inlet.
Like most environmental threats the regulation can not consider all what-ifs as combinations since no engine could be tested to cover all possibilities. Instead for environmental threats, birds, rain, hail, the regulations consider past encounter statistics and set a threshold (in a single ingestion test) that has to be met that is intended to provide a level of safety (factors better than your average flight risk for all causes)