Went to Jurong Bird Park yesterday (by myself, unfortunately).
I'd told myself I would try using my old Pentacon 135mm f/2.8 "bokeh monster" but manual focus just is so clunky. I ended up using the 100mm f/2.8 D-FA Macro, which, while a macro lens, works just fine, although it is a bit too short.
Small Bird (lorikeet, about 4 inches long):
I think it's safe to say only a macro lens could take both these photos!
But that said, the K20D + 100mm Macro has got no chance at all of tracking birds in flight. And 100mm is too short for any except the biggest birds. So for captive birds the ideal lens would be a 70-200mm f/2.8 zoom. Which Pentax doesn't make, and the Tamron and Sigma offerings don't focus that quickly on Pentax bodies..