Do you happen to have a remote control for your camera? If not, I strongly suggest you buy a remote. That way, you can reduce camera shake because you aren't touching anything on the camera when you take the picture.
And I think you should maybe save up for a better tripod. Earlier this year, I was getting so frustrated because I could not get decent focus on the eyes of the birds that I was shooting. I always try to focus on the eyes, but they were always kind of blurry/out of focus. However, I mounted my camera on a tripod and used the shutter remote to focus and to take trip the shutter, and I have to say that I noticed a HUGE improvement.
So try to see if you can buy a remote control and then a better tripod. With those, you may be able to get the birds more in focus, allowing you to crop it without it looking blurry.