Egg shell changing colors
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As you say you are not sure it is the same bird, how are you sure it is a BR producing the eggs? Crack one and post a photo of the internal shell and also post a photo of the birds in question. External shell coloration can vary throughout a hen's laying cycle, but the internal shell color will not vary as brown layers lay a white egg coated with brown pigment and green layers are producing a blue shell (internally) that is coated in brown....which is why seeing the internal shell color is going to be key in determining if you are correct in your assumption regarding the source hen of the eggs you have received.