The majority of bird watchers say that 8 x 42 binoculars are the best magnification and aperture combination for bird watching. The magnification is good enough to allow for hand shake, and the field of view is good enough to follow birds through the sky.
Buying binoculars with the highest magnification may not be practical for bird watching. Higher magnification increases the effects of shakiness, making the brain work harder to interpret what it sees. Binoculars with an aperture that is too low let in less visible light and have a field of view that makes it difficult to find track birds.