Interesting stuff, Jev. I didn't know that HSS starts before the shutter opens. Next time I have the opportunity, I'll do some experimentation with manual HSS (no pre-flash) to see what sort of reaction time I get from the White-naped Honeyeaters. (These are a good test bird as they seldom fail to react to the flash, but (at Belinda's place) they are common and one calls in to drink every few minutes, so I can afford to spook them, confident that another one will turn up soon enough.)

You have to be a bit lucky with bird photography all the time! Your method sounds like the way to go. In the sort of location where you are worlking with birds and flash - typically the atraction is food or (most often) water - there are usually only a half-dozen favoured perches, so it is perhaps feasable to set up on one and learn how much + or - to dial in for a couple of others. As it is (using ETTL) you are dialing in different flash and exposure settings for different species and different perches, and get to learn that (e.g.) the spot over on the left needs a wider aperture (weaker backlighting) and a bit more negative FEC (a little closer to the camera), so I guess that it's just one more complexity to get the mind around and the clumsy fingers to deal with.

The 580EX II has an option to shoot using TTL rather than ETTL; I've never used it. Would that have the same effect as using an old flash?

That 3 x number was just something I made up on the spot, but somewhere around a couple of stops ... that seems about right from my experience. You can certainly shoot from a good distance further away, and (using HSS) get up to insane shutter speeds if you are reasonably close - I'm not sure what the fastest I've used is, but up about 1/2000th or so works fine. And yep, you still get movement blur at that speed: to properly freeze a small bird in flight, I like to be at 1/3000th or better - which must be taking us well beyond poor light / flash territory!