A buddhist friend writes, "The reason why Tibetans don't eat chicken:  The rules about taking life are not hard and fast in the sense that the less life you can take the better.  For example, it is better to kill a whale or an elephant than a shrimp.  All three are one life, but the whale will feed a few hundred people for quite a while.  [in fact I heard this very argument from the head of P.E.T.A.  On Hard Talk on BBC!, as well as by HH Shamar Rinpoche one of the 4 princes of the Karma Kagyu School].  As far as pigs go, the pig is considered to be connected with the Kandrolma, or Dakini, Vajravarahi, which means adamantine sow.  So that is definitely a no-no."