Our cuckoo pattern Dominique sports a black feather or two after a molt.
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I'm not surprised BRs and Doms are similar. BRs were bred from Dominiques (
The American Dominique: A Treatise for the Fancier by Mark Fields). We went with Doms to keep the original Pilgrim breed and size and to keep the self-sustaining broodiness of Doms. Plus we have a personal preference for rose comb birds or walnut, pea comb, or no comb breeds.
If BRs have good calm temperaments it's a trait inherited from their Dom ancestry