*Digs up Evolution notes*
We probably just have to observe them for a while. Within this quest, Kars was able to determine the genetic potential of multiple species accurately enough that he could tell they could interbreed with humans. Further we can surmise that Kars is capable of twisting both his own genome (and his physiology) by creating genes whole cloth from the fact that he was able to give himself working avian wings, when the earliest common ancestor for Mammals (I'm assuming Kars is a Mammal at any rate) and the birds and reptiles was 310-320 million years ago. Since the earliest flying reptiles from which birds evolved didn't arise until 150 million years ago in a completely different branch of the phylogenetic tree, the genes for wings wouldn't have even been present within his genome and he would have to create them to give himself said wings.
And Cell thinks he's hot shit because he can absorb other creature's genes.