by bird do you mean ornithology? if so can you describe your typical shot and also extremes that you will likely encounter? thanks.
I recommend getting the sigma 150mm-500mm os
it has the optical stabilization as well as being 255mm-850mm on your camera.
has ultra fast focus especially in the high focal length registers.
comes with a lens hood, case, and a tripod mount.
300 mm will definitely not be enough to get close to the action.
its heavy but it certainly well built. make sure you buy the 5 year warranty its worth it especially with such a big investment.
http
/www.bhphotovideo.com/c/product/549255-REG/Sigma_737101_150_500mm_f_5_6_3_DG_OS.html
this is what i would buy. im a pro photographer and love canon l glass but hate the cost. sigma lenses are built like tanks for the price and are extremely sharp. they seem to resist wear better than canon's as well... they have a nice matte finish on them which provides nice grip. still, a lens a delicate piece so protect it.
the 850mm equivalent will get you as close as you will ever need to get so you can pull out as in some cases you can lose sharpness.
get the 5 year warranty in case it fails on you lenses do that from time to time. its happened to me but never with a sigma lens. still get it. any questions ask me.