The DA* 300mm f4 is very nice, I hear. But if you're asking, like me you can't afford it.
You'll have to give up something to get a better price. I got the SMC Pentax 300mm f4 - manual focus, manual aperture, 77mm filter and over 2 pounds. This lens and the older screw-mount variations are the only non-* Pentax 300s, so it's cheap (sort of). It's faster than the zooms with better image quality. You may get some CA in high-contrast edges, also a problem with the zooms. It has an integral hood.
I liked the Tamron 70-300mm f4-5.6. It's not as good as the Pentax but you get a zoom, AF, auto aperture, even 1:2 macro. And it's a convenient size and weight. I tested these two and some others I didn't like as much here:
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/98808-300mm-le...p-options.html
The only other lens I've used is an older Sigma 100-300mm, I think f4.5-6.7, 55mm filter, plastic mount. It's small but kind of sucks. There's an equivalent Quantaray, still bad, except with a metal mount.
I think birds are difficult in many ways. Here's a small bird with the K300/4, right at its minimum focus distance. The photo is uncropped, through a window, f5.6, 1/125, ISO 1600, on a tripod.
The window hurts contrast a lot. Anyway, although you might have a brighter day, small birds are rarely going to fill the frame even at 300mm. Surfers might be far away, but conveniently are larger and probably in better light.