Since there are proghorn and muley's on Antelope, they may also look longingly at castle ROCK and dance across the mud flats, especially when the mud is frozen and not crawling with brine flies. So you are probably right, some antelope scat. I am not sure I want to walk 10 miles, but the water situation intrigues me too.
Now that I look at the internet, google concurs.
I also did remember correctly, there were boars on the island back when it was private. Now if one of those boars had been hiding in that hunting cabin, say reading some old magazines from the 60s and 70s that had been stashed up on the ledge between the walls and the roof and turned to see a kilted figure at the open door, that would be something else.
If that bird had just perished on the island, then it could be the resident rodent population that ate the body?