2 ducks limping with no signs of injury.

[Rory the Chicken]

2 of my ducks today have started limping, they are a Khaki Campbell and a Silver Appleyard. I first noticed the Silver Appleyard limping and then later today the Khaki Campbell started limping. I checked both of their feet and all seemed fine, (no obvious signs of Bumblefoot and no swelling). Does anyone have any idea on what has happened to them and is there anything I have to do to help?
Any help would be much appreciated.

 

[casportpony]

Can you post a video of them walking?