One unexpected problem that I ran into in getting so close to the subject (profiled bird swimming left or right) and using the center focus point on it's head/eye, was cutting off it's tail, while the other half of the frame was empty. In situations like this I'm either zooming out and/or trying to select different AF points to use as much of the frame as possible, but it doesn't always work well. I haven't tried locking the focus and moving the camera on a slow moving subject to take advantage of the entire frame, but I suspect it wouldn't work very well. Any thoughts or advice?
I am so sorry that i didn't respond to this a few years ago when you posted it. . I just didn't see it.
Yes, if you are close enough to the subject, there will be many times when, if you use the center focus point on the bird's eye, the tail will be cut off. . The thing to do is to use a different focus point. . If the bird is swimming (or flying) from left to right, then you want to select a focus point that lies on the right part of the frame.
Of course, you'll want to be in AI Servo focus mode, and have the back button activated and the shutter button de-activated from focusing duties. . When I am shooting in these types of situations, the fingers of my right hand are continually moving as they work the joystick to change the active focus point. . Every time a duck turns its head slightly, or swims another couple of feet, or another duck enters the frame, then you'll have to rapidly change the focus point so that it best fits the new composition.
And of course, all the while, as one is trying to keep up with the bird's continually changing position by changing the active focus point, one will also be zooming in and out, so as to best compose the shot in the camera to avoid cropping later. . And you'll also be keeping a close eye on the background to make sure that there isn't any little thing behind the bird that will be distracting. . So there is a whole lot of stuff that your fingers and hands and eyes and brain are doing all at once whenever the subject is close enough for good images.
This kind of bird photography is not relaxing at all ...... in fact, it is quite stressful because you are trying to do so many things so quickly, and if you do anything just a little bit wrong you are missing shots. . But the results are worth all of the stress and hard work.