Swollen Crop In Duckling

[Rutilant]

Hello, I've recently noticed my mallard duckling male has what looks to be a swollen crop. I am not entirely sure though, and I wanted to get an extra pair of eyes on the situation. If it is, what can I do to help? It's been this way for a few days. It's hard to get a good picture with him, he's a bit skittish.

 

[chickens really]

Does it only eat dry feed?....Has it got food out at all times?..Have you given it anything other than feed?...They need grit to help grind down food in their crop....