I have a strong feeling that he was attacked by one of the two big domestic ducks that hang out at that spot, probably the blue Swedish. The only reason why Pepper and his brother were even there was because their mom, Peaches, is in the area.
It appears that whatever happened affected the elbow or just above it. I didn't see any broken bones protruding and it may be a tendon or joint issue. He may never fly again.
He doesn't seem to be in any pain or suffering and even banged his wing against a few objects without flinching. Since he is a hybrid, he is usually not accepted at wildlife rescues and human societies. Hopefully, his wing will get better.
The two large domestic ducks (friends of the buff that died) were totally missing today. It is strange that they both disappeared at the same time. It would also be strange if they were taken by someone for dinner because those two ducks were starving and didn't have much meat on them. If someone who is reading this rescued them, please let my know in the comments box.
Below is a photo of a courtship feeding between two Forster's terns. This is the best photo I could get.
Also, here is a photo of a western grebe that got close enough to let me take a photo of him, though it was through the reeds. A Cooper's hawk frightened him and he dove under water.