The largest water snake I ever saw or found was a 5 foot plus Florida water snake which was located quite a distance from its canal, but just feet from a busy road. When I found the snake in the tall grass, it had just finished consuming something very large. The bulge looked like it had just swallowed something between the size of a soft ball or bowling ball. I figured since the bulge was not long or narrow, it must have been a very big rat it had eaten.
The snake could not move an inch as it was totally exhausted from eating or fighting its prey. Since it was just a few feet from the highway, I mistakenly thought to help it and picked it up by the tail a few inches off the ground and pulled it toward the canal to help it back into the water. The snake began to regurg and although I let it go right away it continued to throw up.
Out came an adult red nosed coot or some bird of that nature. But the bird was big and looked freshly killed. I did not notice any scars on the snake, however the bird could have been hit by a car and the snake could have found the bird dead or injured and ate it? But I figure the ants would have been on the bird, but who knows as this was over 25 years ago and we did not have fire ants like we do today. Today, the bird would last 10 minutes before the fire ants took over the bird and the snake.
I have seen other water snakes eating carrion in the wild such as dead rotting fish and rodents but never saw a bird being taken. I did see a cottonmouth scraping, and I mean scraping with its mouth and pulling off a piece of smashed stuck on hair from a coon that was killed days before. Also, the cotton could not see me as it was dark and my flashlight was in its face, so I handed it a big dead frog that was nearby. The cotton could not strike and did not want to as it was overcome with the sense to feed. It ate the frog and continued to look for more road kill but I scared it off the road eventually.
I think I saw one time on television where this water snake ate a baby alligator. That most likely was set up by someone, although it seems like it would happen in nature. And I believe that most wild snakes will eat just about anything if hungry enough and some in captivity have proven to do so to some degree.
However noted, chicken is known for developing several toxins that if not kept at the right temperature could kill or sicken humans. As such, if you give a big chicken dinner to your water snake, the chicken could possibly develop these toxins and hurt your snake if not quickly digested. I would keep it down to small pieces. And I bet that chicken poop your snake is going to dump in a few days is going to make you throw out all the chicken in the house.