When playing Seasons Episode 12 (Piglantis), every level has a waterline. You have to have 500 birds fall into the water. It doesn't matter if they enter the water 'in flight' or just fall into the water after hitting something. (I got this achievement after the blue bird split, hit a structure and fell into the water.)
You probably won't get it just by going through the levels normally, but probably will if you go back to 3-star every level.
EDIT: The last title update has added a counter for this stat to the bottom of the statistics page, but you have to scroll down to see it.