Breed/sex of bantam chicks
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Are the on a medicated feed that contains Amprolium? Amprolium helps prevent coccidiosis which can sometimes stunt a chicks growth. Also being different breeds the one chick could be a bantam which in my experience don't grow as fast as large fowl. Being different sexes can but I don't know about that much of a size difference; could but I'm not putting full belief into it just because they are different sexes just yet. As for sexing them you need to stretch the wing out to show how the shape the secondary feathers make as a hole. Do they stay together leaving a gap between them and the primaries or do they flow right to the primary feathers kind of spreading out? It's still early to be very accurate but can still give an educated guess. As for that black chick I'm leaning towards cockerel. Post again a couple days. Try to get pictures similar to these.