"What do ducks eat?"
Translation:Co jedzą kaczki?
I could be wrong, but I seem to remember jak-- is used to ask for a type or an adjective (e.g. Jaki chleb lubisz? Which bread do you like?). Whereas "Co" may be asking what do they eat, in general. Perhaps an answer to this question would be "ryby" to which the follow up question of "jakie ryby?"