I have often seen photos like this and simply assumed that the feet had been edited out for some reason. That is not the case, this mallard has his feet tucked into his feathers to keep them warm. If you look closely you can see a wrinkle in his feathers where he has hidden his foot.
This is an example of a photo where I could have probably used a lower ISO and/or smaller aperture to slightly improve the photo. If you examine the exposure information which I have included you will notice that this photo was taken with an un-necessarily fast shutter speed of 1/8000s. I could have used ISO 400 and the shutter speed would have been 1/4000s and still fast enough. I could have also adjusted the aperture one full stop down to f/5.6 and still had a shutter speed of 1/2000 which would have been more than adequate to freeze the motion.
ISO 800 – f/4.0 – 1/8000s – 420mm – Handheld – Natural Light
Canon EOS 7D Body and Canon EF 300mm f/2.8L IS USM Lens and 1.4x Canon Teleconverter MkII
Photographed in Eagan, Minnesota on 11-16-2014 at 2:30 PM