Birds are members of a group, saurischia, best known for having "reptile-like" hips. Even though not all members of the group have that characteristic, that doesn't affect the validity of the group. In cladistics, which is by the far the dominant approach to taxonomy nowadays, taxons should be monophyletic groups or "clades" comprising a common ancestor and all its descendants.

There is current debate and research as to whether the traditional saurischia is a monophyletic group or not, and some classifications put ornithiscians as closely related to theropods with saurischia either redefined without theropods included or not used at all. But bird anatomy does not by itself invalidate the traditional view.

https://en.wikipedia.org/wiki/Saurischia