Raise the feeder. The bottom where they get the feed from should be slightly higher than the level of their backs so that they have to reach up a bit to get at it.
Some feeders do not have much of an internal lip in the bottom pan. This helps to prevent them from beaking it out. If yours does not you may want to look at some other models to see if you can find one better.
Some feeders do not have much of an internal lip in the bottom pan. This helps to prevent them from beaking it out. If yours does not you may want to look at some other models to see if you can find one better.