Frank,
I have gotten to the point where I handhold a lot of my flight shots. I find the movement of following the bird is more natural handheld vs on a tripod. On a tripod with gimbol mount your body moves a different direction than the bird is flying, where holding by hand you swivel with the bird.
One thing I also do on blue bird days is shoot with centerweighted metering and perhaps -.3 ev that keeps the blinkies or blown whites down on my white birds, even during midday sun
Lots of flying white birds here on my family site shot that way:
http://homepage.mac.com/wadedowdy/Photos/PhotoAlbum61.html
note the gull flip sequence was shot matrix metering, and you can see the blown whites, but this was nearly 4 years ago, and i have learned a whole lot since then,
and some on my select site too, but not as many flying birds, but lots of white ones!
http://homepage.mac.com/wadedowdy/photoart/PhotoAlbum40.html
I generally shoot white birds either with spot metering metering on the white and +.7Ev to keep them white instead of grey, or I shoot in center weighted and leave alone or minus -.3
sometimes I do shoot matrix and -.7 to -1.0 EV.
Cheers, and these are good shots! work out that 300!
Wade