Margot Robbie’s “Birds of Prey” is set to open well below forecasts, with about $34 million at 4,236 North American sites, early estimates showed on Friday. Estimates for “Birds of Prey,” a spinoff of 2016’s “Suicide Squad,” had been in the $45 million to $55 million range, with Warner Bros. projecting an opening closer to $45 million. “Birds of Prey,” which carries a reported $80 million price tag, touched down with $4 million on Thursday night. Reviews have been strong, earning the film a 84% rating on Rotten Tomatoes — a reversal from “Suicide Squad’s” 27%. The film made $7.8...