I was told by Jamie (APA judge and curator of the zoo with a degree in animal science) That their protein level should be 18-20% at all times. He pretty much said my 15% grower was not cutting it and I really needed to up it if I wanted to hatch healthy chicks and get the best out of my birds.
Yes he says layer is about the worst thing you could feed your birds. Not so much because of the calcium (which roosters CAN handle when it's breeding season - it helps their fertility) but because it is formulated to feed your birds the minimum nutritional values to get eggs. IE: Don't feed this to your breeding flock and avoid feeding it to your laying flock as well.
I had just started game bird breeder for the birds in the barn. Made for breeding season. 20% protein (animal protein) and the babies were still on starter (18%).
They don't have an all flock around here. I will stick with unmedicated starter until next breeding season.
He studied nutrition and is very smart when it comes to these things. I'll take his word for it

His birds are the picture of health. His main breeds of focus are: Toulouse Geese, Silkies, Modern & Old English Game Bantams.
Wow I got distracted and didn't post this.