In death, the Fire Crows looked like nothing more than robust Ash Crows.
The two were of the same species to begin with; it was just that some Ash Crows were born exceptionally stocky and well-developed. Having nurtured spiritual essence, they could trigger the latent bloodline abilities within their bodies, thus mutating into Fire Crows.
The strongest individuals among the Fire Crows could even gain a measure of intellect, cast spells, and become Demon Spirits.
Chapter 223