It was a wild bird trying to fly for the first time that landed in the coop. He tried to kill it and feed it to his ladies.
A wild baby bird is not the same as a chick that's slowly introduced to the flock (and yes chickens have killed and eaten wild birds).
Like some of the folks above I also do early integration. My current "babies" were brooded outdoors and moved in with the adults at just under a month. Here's how I do early integration: https://www.backyardchickens.com/articles/short-on-time-recycle-a-prefab-brooder.73985/