Nestling or Rearing Food can be bought at a pet store. You feed it to the parents in a dish, and then the parents feed it to the chicks.
Nestling food is made in a variety of ways. Many breeders mix mashed hard-boiled eggs with breadcrumbs and grated carrots.
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Other breeders bake a poundcake with lots of eggs. When cool, the cake is mashed or blended into crumbs.