I had a look at your hawk shots, in fact a couple of views. And one of the first things I note on the couple that have exif available is that under the lighting conditions you had you didn't use any exposure compensation though the birds are well exposed on top and under the wings.
In all of the shots except the first this wouldn't be expected unless you pulled the exposure up in post. And from the look of the noise it seems this is what was done.
You had a lot of light & high shutter speeds but grainy images are indicative of a heavy hand in pulling up shadowed areas. Also it appears these 4 shots were all in sequence however the sky color changes in each shot. This would make sense if the subject was circling around the photographer but I am thinking that a liberal use of the saturation of colors was used, again in post if you has pulled the shadows up too far and had the sat. adjusted too much it only adds to the noise.
In post production it rarely pays of to make significant adjustments to the exposure, as a RAW image, initial exposure adjustments anything pushing up to a couple stops can create issues down the line in post and as jpeg you have maybe a quarter of that latitude. So getting it close in camera is imperative.
If you are shooting in AV then using exposure compensation must become 2nd nature, bright skies and getting the subject exposed well often means being over 1 stop to properly expose the subject and still the shadowed areas will have shadows. Go plus on the wheel to pull up an underexposed or white subject, minus to keep highlights from being blown.
You are using spot metering which does help if one can keep the bird filling the center circle but if it falls out then the metering can be quite a ways off. As can shooting flight shots in a burst mode, if you acquire the subject and just hold the shutter down the metering is only for the initial lighting, as the bird moves across the sky and different lighting condition you set with the first shutter press.
If you are shooting jpegs I would have the camera set to a neutral pic style, this allows the most latitude for adjustments in post to get the colors to what was seen without clipping any particular channel.
I would also forget about using the incremental ISO stops, on the 50D the noise difference between 500 & 800 is insignificant, and if this is the case why not use all of the shutter speed or additional dof you can get?
It pays to have the AF and shutter button (metering) separate and keep the AF running on the subject but have a brief pause between shots of the subject to allow the camera to acquire a new meter reading. Instead of a machine gun burst try a few double taps and see if that doesn't improve the exposures.
Photography is about capturing light, I often stress this when people ask for advice. Doesn't matter if one is shooting birds, landscapes or wedding images a great shot always comes down to the lighting first.
And while it isn't the best conditions to shoot in harsh light sometimes one must do so. In these cases it really pays to understand what the camera is "thinking" about in each metering mode and how to make these different tools work for you instead of against you. Personally I use spot on occasion but am defaulted to partial on all of my bodies. Spot has it's place but unless one is sure exactly of what the scene presents and more importantly has the time to consider it can be sporadic in results.
Sometimes it means just passing up on a shot unless one can get moved to different position, throw some light from a flash or shoot for something other a than representational image.
