Hi Gunther,
I've often found that if you photograph in the middle of the day, you can take 5 images and they will all have different clarity. The disturbance of the air ( "heat waves") can leave one picture perfect and the rest looking poor. In the case of your image, which is a very nice composition, this doesn't seem to be the problem. You literally do get a wave of distortion running through the image in most cases when this is happening, and not in just the one spot. The ripples across the picture are clean., the leg is not.. So what might the problem? If you let us know your camera settings, this would be easier to suggest. At a guess, I would put it down to your shutter speed.
I've had pictures taken at 1,600th of a second which weren't too clear despite the camera panning with the action. Often small amounts a motion can be hidden with sharpening after the event, but here, in the case of the bird's leg, I don't think you will find it very effective. The most extreme parts of the bird (wing tips and legs) are where this is usually seen. I take a lot of images of small bush birds, and the advantage with them is that you can get away with a low ISO, which gives you great colour and contrast; and a very slow shutter speed - say a 60th of a second. It's because the birds often stay still while they listen and look. Shore birds aren't into this sort of thing nearly as much. They are generally moving about or are asleep.
The first thing I do when I come to a place is check what light I have and then decide what I'm going to do about it. If the scene is variable, I won't consider shooting manually. This includes BIFs. My camera is a dam sight better at picking exposures than i am when you are panning across a variety of backgrounds. Having said that, I use either aperture or shutter speed priority. For most things, I find an aperture of 6.3 suits my purposes. The background is nicely blown, but I have a pretty decent depth of field. If it's BIF's or fast moving small birds, I want the shutter speed to nail the image and I crank it up.
My suggestion for you here is choose a high shutter speed and let the camera choose the aperture and ISO. The take a lot more pictures than you think you might need. If you choose to be up sun rather than shooting into it, you may find the lighting a little kinder.
Early in the day you'll find the birds are keen to get a feed and will tolerate you far better than later in the day than when they are full. The light is better then too.
Just some thoughts about you picture. I hope they are a at least little useful. You have a great eye for composition and I'm sure you'll soon find yourself taking great shorebird shots.
Cheers,
les