Burger King and Tim Hortons are struggling with a staffing crunch and the Delta variant keeping coffee-loving office workers at home, causing parent Restaurant Brands International Inc. to miss estimates for quarterly revenue on Monday. Restaurant Brands also faced stiff competition from McDonald’s Corp. and Wendy’s Co. doubling down on marketing and launching new menu items. “COVID-19 contributed to labor challenges, which in some regions resulted in reduced operating hours and service modes at select restaurants as well as supply chain pressures,” Restaurant Brands said in a statement. Burger King, like most rivals, has struggled to ensure its restaurants have sufficient staff, with its newly launched hand-breaded chicken sandwich also considered a labor-intensive product. Wendy’s launched a new ‘Big Bacon Cheddar Cheeseburger’ and reformulated its french fries to keep them crispy for longer earlier this year, while McDonald’s collaborated with boy band BTS and rapper Saweetie to draw customers. Same-store …