There's one more factor- free range. Silkies are horrible mothers if free range. They almost always lose over half of their brood. They really show their uhm, "stupidity" in this regard.. Don't think banty cochins would do well either, especially if hawks are a problem.
OEG is hands down the best.. but even they have a draw back- many are way too protective.. either attacking any person who dares look their way within 10 feet OR shrieking like a banshee if any person dares enter within 20-30 feet of them.. scattering all the babies and teaching them to be terrified of all people.
It is very true that many mixed breeds, does not matter much of what breeds they were of often are excellent mothers.
However, almost any standard sized hen that will go broody will turn out to be decent or good mothers, if the hen is isolated while broody and raising chicks in a coop by themselves with the occasional turning out to free range for the day. I have a big extremely heavy(she has the what people call "deep chest" but to me it just looks like a horrible case of beer belly- sorry folks..), usually clumsy cuckoo Marans hen.. looking at her you would not even be able to imagine her being a good mother to an ostrich baby. But.. she's the most caring, doting, spoiling, careful mother.. every year I give HER my "most precious" peafowl eggs to hatch and raise by herself. She's that good.
