Tamron 70-200 f2.8 or Canon 70-200 f4

[JeffKnob]

I have a Rebel XT and am wondering which of these would be better for my situation. I purchased a Tamron 18-270mm lens thinking that I could have a good all around lens. I am struggling to get enough light in unless the sun is out. I want to be able to take some pictures of birds and animals. When zoomed out all of the way the the aperature can't let enough light in. I already have the iso at 1600 and the shutter speed low. When trying to take pictures of flying birds the slow shutter speed isn't going to cut it. My thought is if I were to exchange the 18-270 lens for either the Tamron 70-200mm with f2.8 or the Canon 70-200 with f4 both non IS. I mention these two lenses because they are available to me and cost in the same range as the 18-270. Thoughts?