Are modern birds reptiles?

It sounds as if you definitely do know the difference between taxonomy and cladistics. Let's take the cladist concepts further. Mammals are derived from reptiles too, so we have now sunk two cles. However, reptiles (now including birds and mammals) are derived from amphibians, so we would have to sink the cl Reptilia as well. And ampbians are derived from fishes. Taken to its logical conclusion, the cladists would have us consider all life a single species. Not very useful, is it?