Remember, even with image stabilizing lenses, you really need to shoot at the fastest shutter speed possible while keeping the ISO setting as low as possible to reduce digital noise in your images (higher ISO = faster shutter speed in low light but higher ISO = grainy, affected images).
Using a tripod is really critical. If you want to know why, try holding a laser pointer at a wall from 10 feet. Watch how the laser dot bops around, despite your best efforts to hold it still. Now, if you're hand holding a camera that weighs several pounds, and your tiny bird subject is in movement, your hand movement (remember the laser pointer analogy?) and the bird's movement will result in less-than tack-sharp photos; you'll get blurred images. I carry my camera, set to the lowest ISO for the ambient light, ready to shoot over my shoulder while mounted to the tripod. If I see a bird, I carefully bring the tripod down to the ground and set up and shoot, generally in seconds. Take caution, however, if you decide to carry your tripod over your shoulder with the camera attached. I use a very high quality tripod (Really Right Stuff) to ensure that I can do so without losing my camera. If you carry your camera mounted, all ready to shoot, make sure you have a safety strap around your neck or some other point of safety. Failure to do so may result in your camera suddenly detaching and crashing to the ground! Frequently check camera-tripod connections.