Some birds are cluey brutes! Grass seed will be ok BUT we used to have trouble when we were planting corn with cockatoos, we would plant the corn in rows about 2-3inches deep with the seeds around 4 inches apart and the cockies would find a row and then work their way along only digging a hole every four inches, they would dig out that seed and then move on and get the next one.
I never really worked out how they knew where the seeds were but it was super frustrating as only a handful of cockies could clear a fair little patch in a short amount of time.