The girls felt rather curious about the bird which Camila killed a moment ago, spirit beasts have their own way to cultivate, instead of a dantian they developed a beast core, which should be inside their bodies not incrusted in their chest like the black crystal, which after being purified by Aster turned into a yin accumulation crystal.
Originally, they thought it was something like the crystals which the lizard used to generate those flames, an evolution trait caused by the environment in which the spirit beast has lived, but the lizard did have a beast core, according to Felicia.
That bird on the other hand didn't, instead of that it was using an impure yin accumulation crystal as its core, which was weird.