I would like to thank you guys very much - you have been absolutely brilliant!
The main thing this thread did was relieve me of my iso-anxiety. I upgraded to k5 mainly to get less high-iso noise, but I was still afraid to go over iso 800 - when in fact I should have feared the max aperture of my Bigma and motion blur a lot more. TAV makes SO much sense, and it is my program-of-choice for bird shots from now on!
(I'm particularly baffled by the difference it makes to use f8)
This is more like it, right (f8, 1/500s on TAV):
http://dl.dropbox.com/u/10485935/Gr%C3%A5g%C3%A6s.jpg
mickey: You might be right about the legs of the egret being sharper than the eyes, and I should do a careful and systematic investigation of the need for focus adjustment. I just didn't get around to it yet (tried a bit of trial-and-error-adjusting, but that didn't help).
audiobomber: I have a Pentax 1.7 converter with AF, and eventually I would love to combine that with a DA* 300mm f4, which would be cabin-luggage-sized and might be almost weather proof with a bit of sticky tape around the converter...
It's just outside my budget for now, and obviously I have something to gain by improving my skills with my current equipment.
Chex: Thanks for the tip, but I do disable SR and use the in-lens stabilization. I would love to send it to you for QC testing, but it weighs about the same as a small lorry, so I couldn't possibly afford the postage...
A couple of you asked about the car engine: It was off. You are right about it being windy though, and the heron wasn't motionless either, and I've learned my lesson - 1/500s or above!
Monopod is usually not an option - too much gear to carry already, kids, spur-of-the-moment-shooting... but I have a steady hand (honestly) and use tree trunks, car windows etc as often as possible...
Again, thanks a bunch! I hope I replied to most of the questions - if I missed any I am sorry, but there have been so many useful replies, and I did read and appreciate them all!