Surely to save time and considerable effort, the question has been put on this Forum, so as to gain knowledge that which others have spent years gaining- a short cut if you will. The point of these Forums is to share knowledge... Although maybe for some it is to show off without ever actually sharing?
As you can see on this very website for the money that Cabbie wants to spend it's not worth looking at Olympus or Pentax- so to the other three that you mention
Actually, I'll have to disagree - I can't see any such thing on this website. In fact, I see the Pentax KX being very well equipped and quite capable for a very reasonable price. Olympus I'm not as sure about - I merely suggested checking them out to find out if they have anything that might work.
... D3000 is better than anything else at it's price point, D90 is not far from being replaced hence the lower price (currently almost as low priced as the camera in a lower class from other brands) . The nearest Canon to the D90 is far more expensive 50d and the Sony's aren't as user friendly/ergonomic... So as to Cabbie's original quest for a short cut to others knowledge... The short cut is- Get a D90 mate its not about brands, its just that at the price your looking at there isn't anything better at the moment.
Commenting on Sony as not being as user friendly or ergonomic is a presumptuous thing indeed - by point of fact ergonomics are very much a personal thing and it should be up to the individual to decide what is or is not a good fit for them. Possibly to YOU, all Sony cameras are not good ergonomically (though puzzling, since the ergonomics are drastically different between Sony models - the A330/380 vs A500/500 vs A850/900). For someone else, they may be just right. The same goes for Canon and Nikon. Again speaking to my primary message - decide for yourself, and don't always feel forced to buy what someone else likes.
As for offering knowledge - absolutely I am willing to do so. In fact, I've recommended Pentax cameras, Canon cameras, Nikon cameras, Sony cameras, and Olympus cameras to people who've asked - all based on their stated needs, wants, or likes. In this case, the OP didn't offer any specifics that would exclude any of the above brands in my opinion. If the OP were to declare that video was an absolute must in a camera, I'd leave Sony out. If they were to say they wanted a quick focusing live view system, I'd recommend primarily Sony. if they wanted a tiltable LCD screen, I'd recommend Sony & Nikon. If they wanted weatherproofing, I'd lean towards some Nikon & Pentax models. If they needed a specialty lens only offered by one or two manufacturers, I'd recommend those manufacturers.
But for needing a camera under $1,000 that can serve as a decent birding camera, with fast focus, good burst speed, and reasonably priced lenses available, I personally believe a Pentax KX, Sony A500, Canon 500D, or Nikon D5000 could all serve just fine for around $500-700, which is competitive with a sale price of a D90. and since there are some very different ergonomics and very different feature-sets on each of those, it seems valid to recommend that the OP consider them all.
Again...just my opinion.