B!
Thanks for asking.
In my humble understanding...

Meat forbidden is: (impure/unclean/etc)

The flesh of pigs.
The blood of any animal.
Animals that have been killed before the actual cutting of the throat ( roadkill, when another animal has claimed it for themselves, certain slaughtering methods )
Any carnivours are forbidden ( dogs, cats, tigers, birds of prey, bears etc.)(and all body parts for that matter)
( so an answer to your question about buzzards is yes they are forbidden, they eat dead meat, and are impure, God knows best if I have made a mistake about that, may I be forgiven, amin)

As far as shellfish, this is a misconception in Islam in some cultures.
It clearly states in the Holy Quran that anything from the Sea is Lawful. ( waterbreathing animals)



{Forbidden to you (for food) are: dead meat, blood, the flesh of swine, and that on which hath been invoked the name of other that Allah, that which hath been killed by strangling, or by a violent blow, or by a head long fall, or by being gored to death, that which hath been (partly) eaten by a wild animal, unless ye are able to slaughter it (in due from), that which is sacrificed on stone (altars); (forbidden) also is the division (of meat) by raffling with arrow: that is impiety." (Al-Maidah # 3, Holy Quran)}

And Allah knows best, and Peace & Blessing be upon His Messenger Mohammed and his companions.

http://www.islamicconcern.com/default.asp

http://63.175.194.25/index.php?cs=prn&ln=eng&QR=210&dgn=3&dgn=2
http://www.eat-halal.com/halal.shtml