Counter-intuitively, shutter priority mode is often not the best way to go when photographing birds in flight. The auto and semi-auto modes often get confused and meter for the sky.

A good solution is to set your camera to Manual, point it at the grass at your feet (preferably sunlit grass, not shady) and set the camera to expose that correctly. Adjust the aperture and ISO so that you get a good fast shutter speed, at least 1/500 (keeping the ISO as low as possible - open the aperture first).

You should find that the birds will now be correctly exposed - you can always tweak the settings a little (you might overexpose a little shooting white birds). The point is that using Manual mode takes away the possibility for the camera to incorrectly expose the shot.

Focusing is the tricky part. If you have a fairly run-of-the-mill lens, AF may not be quick enough to cope. If you have dynamic focusing mode, try using that as it will attempt to track the subject from one focus point to the next. Use Continuous AF so it constantly adapts to the subject. Otherwise it is probably just a case of using manual focus and being persistent; digital photography makes things like this much easier (not to mention cheaper).