Since I mostly shoot birds I think it's best to know where and what you would like to shoot since that will influence lens choice. If you plan on shooting the little guys in the wild, not from a blind then I'd say you really want nothing shorter than 400mm to get really good shots. Sigma made a 135-400 as well as a 175-500 in the PK mount and if you can find one of those it should be under $600. Other than that it's the lenses you listed above or using something that is MF unless you can find an older 400/5.6 that is AF (have seen a few around). The Tokina 80-400 zoom is decent for birding as well. Or you can get an AF 80-200/2.8 and put a 2X converter on it for a 400/5.6. If the lens is sharp enough it will likely be as good as a 400/5.6 AF lens at least ( I know my old Tamron 80-200/2.8 with a 2X TC is as sharp as my Tokina 400/5.6 but not as sharp as my Tamron 400/4). Just to throw out another option, an AFA will give you 1.7X the reach and AF on older MF glass which makes getting longer reaches more affordable. But I have found that it doesn't play well with all lenses, or at least mine doesn't.
Because you can really never have too much reach in bird photography I have gone a similar route to Hans though instead of the Q I bought a Nikon 1 series V1. 2.7X crop factor makes it about 1.8x the reach of my Pentax. From what I have seen so far the pics are better than I can get from the AFA and a lens on my Pentax which is about the same focal length as the lens alone on the V1.
Depending on where you live, there is an easier way to get into shooting birds and that is to start feeding them. Set up some feeders and a few nice perches for the little guys to wait on and you will have tailor made bird shots. Put up a crude blind and you will have plenty of reach with the lenses you have.
Hope this helps some. If I were to choose one affordable lens for birding ( I won't count the Sigma 300/2.8, 500/4.5, 300-800 or any of the 400mm+ Canikon lenses) I'd go for the Tamron 120-600. Alas it isn't available in a Pentax mount.
Last edited by jelow1966; Jun 4, 2014 at 12:14 AM.