The average tenure for chief marketing officers working for the biggest brands in the U.S. fell for the first time in a decade, according to new research from executive search firm Spencer Stuart.According to the firm’s annual study of CMOs from 100 of the top U.S. ad spenders, the average tenure for marketing chiefs fell to 44 months as of 2015, down from 48 months in the prior year. Spencer Stuart also found that nearly a third of the study’s CMOs were new to their job in 2015, the highest portion since Spencer Stuart began formally tracking CMO tenure in 2004.The higher rate of turnover and short tenures are being driven by factors such as retirements, a record year of mergers and acquisitions when duplicate roles are often eliminated, and decisions by CEOs and boards to change marketing chiefs, said Greg Welch, a consultant in the Spencer Stuart Marketing Officer Practice. The mandate of the CMO has changed dramatically in recent years as brands look to reach consumers on digital platforms and leverage big data and new technologies in their advertising efforts.“The intensity, the pressure and the demands on CMOs is admittedly unlike I’ve ever seen in my career,” said Mr. Welch, who initiated the company’s tenure study.The median tenure, a metric that is less skewed by outliers, among CMOs in the study also fell in 2015, slipping to 26.5 months from 35.5 months in 2014.But since Spencer Stuart’s first CMO study in 2004, the average tenure of CMOs has nearly doubled. The average was 23.6 months in 2004 and had increased every year in Spencer Stuart’s study except in 2006.Among the 100 CMOs covered by the study, 30 were new to their job last year. In comparison, there were 22 new CMOs appointed in 2014. Twenty-two of the newbie CMOs in 2015 had never held the title before.Changes in the marketing head of the C-suite have important implications for agencies on Madison Avenue. It is common for new CMOs to make major changes to a company’s marketing strategy when they come in and review the agency roster. A new CMO may also mean the upheaval of the company’s internal marketing department.