The answer to this question can be found in chapter 17. Chuck is explaining to Curly that the plan is to lock up the building site for a full 48 hours. Chuck doesn't want anything getting in the way of his end game. He believes that Curly should be able to keep out any kind of trouble, however weird it may be, that could arise during that time:
"All you've got to do is lock down the construction site for the next forty-eight hours. Think you can handle that?"
"Sure," Curly said.
"No more alligators, no more poisonous snakes, no more stealing," Chuck Muckle said. "No more problems, period. Comprendo?"
Curly then asks about the owls, and Chuck reminds him that the owl burrows are empty. He's lying of course, but that is what he is telling Curly to tell anybody and everybody that might ask.
"What owls?" Chuck Muckle shot back. "Those burrows are abandoned, remember?"
Chuck knows that there isn't a law against his building project if the owl burrows are empty. Curly asks for further clarification because Curly knows that an owl might actually be seen by the general public. Chuck tells Curly to announce that whatever bird was seen was basically anything other than an owl.
"What owls!" Chuck Muckle practically shouted. "There are no owls on that property and don't you forget it, Mr. Branitt. Zero owls. Nada. Somebody sees one, you tell him it's a-I don't know, a robin or a wild chicken or something."