You don't have to draw out the scene. It's just me being selfish as a reader, who wanted to see more. But I do think it's a good opportunity to flesh out Birge's role, if you want that.
You could maybe introduce Caval as a rumor and a shadow. When P&B go to the south field, the men can claim to have seen a wild beast on the edge of vision, watching them. They can claim that it's hunting them. It'd be like bigfoot, though - no more than one or two men see it at a time, and they exaggerate its appearance. They attribute evil to it because they're afraid. Then, maybe a week or five days into their work, Caval appears front and center to meet his master. The men freak out, and so do the boys, but tension is relieved when they learn the beast isn't there to kill them. Over the next week or so they all become accustomed to Caval, and his relationship with the boys is established.
Of course, that's a fairly heavy rewrite with lots of additions overwhat you've got, and I don't know if you want that. It's just a suggestion to make Caval fit into the story a little smoother, in my opinion. By lending him an evil start (even if it was undeserved,) and then making him a friend, you associate him with the ambiguity of the supernatural powers as well.