First, look here, and remember this quote:
<<The Thunderbird will still use the name Athlon>>
The thunderbird is an updated version of the Athlon. Thunderbird was the codename used while the updated version of the Athlon was still in development. For all purposes, they (Athlon/Tbird) are the same thing.
You have to remember though, that there are differences between the two. The 'classic' Athlon had off-die cache, and was in slot form. The new Athlons, or Thunderbirds, have on-die cache, and use a socketed form.