Here's why:

* Chordates are a diverse group of animals that share certain key characteristics at some point in their development, including a notochord (a flexible rod that supports the body), a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail.

* Chickens, like all birds, have a backbone (which is derived from the notochord), and they exhibit the other chordate characteristics during their embryonic development.