Crest can show up later. This is from experience of Ameraucana x Spitzhaubens.Nobody has answered, so I will take a stab. From your splash Marans and a blue rooster, yes, it is weird to only get one splash chick out of 12. But not impossible. When we talk about what percentages to expect from a mating, those percentages apply to each chick. So each chick has a 50% chance of being splash or blue. So in theory, you could “roll the dice” 100 times, and still have blue come up 100 times. But yeah, normally we wouldn’t expect that to happen. Keep “rolling the dice”. If you hatch out a large enough number of offspring, your percentages should likely move closer to half and half.
I do have a question. This chick doesn’t really look like it was sired by either an Ameraucana or a Polish. Is it possible there is some other (non blue) rooster sneaking in?
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I don’t see signs of a crest or a beard, although maybe the crest could show up later?