I like how flash gives a nice catchlight in the eyes of animals and birds. I think shots two and three show very subtle evidence of slight ghost imaging, from the combination of flash + daylight. See in shot 3 how the bird's beak has a soft, white outline, the so-called ghost image,above the beak? I think the ambient light was bright enough, and the shutter speed was slow enough, that you got a very gentle ghost image, which hurts sharpness. This is usually only clearly seen at edges of strong lines, like the beak against the darker branch behind it, and almost invisible in areas of soft contrast,like the birds' feathers, but this is the issue I think I am seeing.
A friend of mine once had me diagnose some flash + daylight shots of black-capped chickadees coming in to his feeder and pre-focused photography spot using a 75-300 Canon lens and a flash. Because high-speed sync flash is actually stroboscopic, he had terrible blurring of the birds, and he was unsure what it was being caused by. But your scenario is different on the junco shot (shot #2): you were shooting SINGLE-burst, regular flash, at 1/200 sync speed at ISO 200, and the movement of the bird's beak is shown in the very subtle daylight-illuminated "ghost" image.
I think the ISO, f/stop, and shutter speed are all fairly close enough to a "daylight" exposure that might be cutting sharpness down. I think that shot #3 looks slightly soft for the same reason, ghost image blurring. The effect is subtle, it's not blatantly obvious, but that is what I see definitely in #2, with the beak outline, and I think it's also affecting shot #3.
This is a tricky situation, when the daylight exposure of 1/200 second at f/8 at 200 to 400 ISO is actually enough exposure to create say a "50%" exposure, and the flash is 50 percent or so. I think you need to get the daylight exposure to be less, and the flash to make up more of the total exposure; the problem is that at 500mm, even the slightest movement of ANYthing is recorded pretty large.
