From a BBC News article:

Investigators say they have found evidence of a bird strike on a passenger plane that crashed in South Korea in December, killing 179 people.
The feathers and blood stains on both engines of the Jeju Air plane were from the Baikal teal, a type of migratory duck that flies in large flocks, according to a preliminary investigation report published this week.

Apparently flock size can reach 1,000,000 birds in the areas close to the airport. Quite incredible and no wonder that the pilots were unable to land successfully if they flew into such a flock.