As blood moves down through their legs into their feet, it is cooled by the blood moving back to their body; likewise, as blood moves back up the legs into the body, it is warmed by the blood moving downwards. The blood vessels are located close to each other, which enables this heat transfer, so that even though the legs get cold, it reduces the impact of the cold feet on the rest of the body.



I either remember this from my comparative vertebrate morphology class I took long ago, or I'm making it up. I can't remember which, it being over 30 years ago, but I don't think ducks have changed much in that time.