Netuser: what's your budget? If you plan to stay with Pentax, then a budget is still needed. You can be successful with birding at 300mm, but it takes a different approach and more patience.
Having a 500mm lens is nice, 600mm is even better, but often you want even more. It depends on the subject! Shorebirds and such can be approached with some techniques that will let you fill the frame at 500mm (using a blind). I guess I am saying you need to clarify what birds you wish to photograph and how approachable they are. Most of the time my subjects are not used to human presence, so they move away or readily take flight. Hummingbirds are an exception if you still relatively still, but they are incredibly small at ~7-9 cm from head to tail.
Give us some thoughts/answers and perhaps we could help out more definitely for you...
Regards,
Marc