For all of my bird/wildlife photography I use a 400mm F5.6. I prefer it to the 100-400 because it’s sharper while being smaller and lighter with better auto focus. I aim to use a shutter speed of at least 1/800 to ensure as little motion blur as possible. Make sure your camera’s autofocus is set to continuous; for Canon this is ‘AI Servo’. If you can, try to shoot around sunset/sunrise for more interesting light.