Canon 300 f/4L or Sigma 100-300 f/4 EX
While I cannot comment on the Sigma, the 300 f/4 L IS is an excellent lens for wildlife. Here are two shots of an Australian kestrel
Both hand held (because of the IS factor). Sometimes I find that the 300 is just too close so I then use my 70-200 f/4L.
All the technique in the world doesn't compensate for the inability to notice - Elliott Erwitt