NEW YORK (AP) — The newest stars on the McDonald's menu won't be around for long.The world's biggest hamburger chain is rolling out two menu items that will be available only during the summer months. A seasonal blueberry banana nut oatmeal will be available nationwide by mid-month, and an iced drink called the Cherry Berry Chiller is already available everywhere.In the years ahead, McDonald's is expected to ramp up its pipeline of limited-time offers — which are available for only a few months at a time — to keep customers flocking to its restaurants.The Oak Brook, Ill.-based fast food company began its focus on limited-time offers on a national scale last year after research showed that variety was a top priority for consumers, said Wendy Cook, vice president of U.S. marketing at McDonald's. Previously, the chain's limited-time offers were mostly at the regional level. Seasonal items give loyal customers something new to try, but also bring in new customers who otherwise might go elsewhere, she said."Obviously customers are out there and they're frequenting different places," Cook said. "They're starved for variety."The new focus on limited-time offers comes as McDonald's looks to extend its dominance in a rapidly changing fast food industry. In 2006, the top three fast food chains in the U.S. in terms of sales were all hamburger chains — McDonald's, Burger King and Wendy's. Now Subway is No. 2 and Starbucks is in the No. 3 spot, with sales at both companies growing at a much faster clip, according to the food industry researcher Technomic Inc.For McDonald's, the limited-time offer also lets the company adapt more easily to evolving consumer tastes that are leaning more toward fresh, seasonal ingredients. Since McDonald's operates on such a vast scale — it has 14,000 restaurants nationwide — sourcing the fruits for offerings such as the new the Cherry Berry Chiller can be difficult.Last year, McDonald's had a total of eight limited-time offers that spanned menu categories (including Frozen Strawberry Lemonade, Asian Chicken Salad, Angus Chipotle BBQ Burger); the company is planning to increase that figure this year. It offered Chicken McBites earlier this year.The blueberry banana nut oatmeal (290 calories) and Cherry Berry Chiller (200 to 330 calories, depending on size) will be available until August.McDonald's declined to say what other limited-time offers are in the pipeline in the months ahead. But in a conference call with investors last month, executives noted that the Chicken McBites originated in Australia.Executive said they plan to continue scouring its regional offerings from around the world for items that might have crossover appeal. That's important because developing a new menu item — even if it's only temporary — can take as long as two years.The development time is shorter for items such as the blueberry banana nut oatmeal since the company already had a template with its fruit and maple oatmeal, said Anne Kohlenberger, manager of U.S. marketing for the company's breakfast category.Of course, there are instances when a limited-time offer is so popular that it earns a place in the starting lineup. That's what happened with the Angus Third Pounders, which started as limited time offers in the late summer of 2009.