Serin
Crowing
and grit is essential. All nutrients in the grain will be lost without grit, as they will have no way to digest it. I recommend Mineral grit for them.
Provide grit, but don't worry if they don't eat much.
I keep a cup of mixed grit in my bird's cage, but she hasn't eaten any since I think last July when she laid eggs last and was craving minerals. She's obviously able to digest her food anyway.