(Reuters) - Corporate America’s biggest shareholders have traditionally been content with sharing their views on a company’s strategy privately with management.

FILE PHOTO: Logo of global biopharmaceutical company Bristol-Myers Squibb is pictured at the headquarters in Le Passage, near Agen, France March 29, 2018. REUTERS/Regis Duvignau/File Photo

But now some mutual funds are beginning to rethink their stance, amid pressure from investors for them to justify the fees they charge and a push to boost the performance of their holdings.

Wellington Management Company LLP’s decision last month to speak out against drug maker Bristol-Myers Squibb Co’s proposed $74 billion acquisition of Celgene Corp, calling what would be the largest-ever pharmaceutical takeover too risky and expensive, sent ripples across the investment world.

This is because these tactics have typically been the purview of activist hedge funds like Starboard Value LP and Elliott Management Corp, not a large institutional money manager like Wellington, with $1 trillion in assets under management.

But in the case of Bristol-Myers, Starboard spoke out publicly against the deal one day after Wellington unveiled its stance publicly.

Wellington’s vocal opposition to the deal is the culmination of some mutual funds gradually feeling more emboldened to publicly challenge a company’s strategy, asset management executives and corporate governance experts say.

“There has been a growing chorus among investors who want these firms to speak up. With Wellington speaking up, it is going to put pressure on the others to do the same,” said Lawrence Glazer, managing partner at Mayflower Advisors, which invests with Wellington funds.

In January, chemicals company Ashland Global Holdings Inc agreed to changes to its board after pressure from asset manager Neuberger Berman Group LLC, which has about $300 billion in assets under management.

T. Rowe Price Group Inc, which manages close to $1 trillion in assets, has opposed several acquisitions, including Michael Dell’s offer to take his eponymous computer maker private, because it felt the proposed deal undervalued the company.

Spurring on these funds to challenge companies publicly is the need to show their worth as so-called active money managers, picking stocks rather than just betting on indexes.

At a time their performance has been lackluster and many have struggled to keep up with their benchmark index, they are under pressure from index-tracking funds who are gaining more market share in asset management. These “passive” money managers charge investors far less, in part because they do not need the army of analysts and portfolio managers to make investments.

“More funds are willing to agitate in search of returns,” Mark Shafir, Citigroup Inc’s co-head of global mergers and acquisitions, said on Thursday at the corporate law institute conference organized in New Orleans by the Tulane School of Law.

RAMPING UP PRESSURE

Despite their deep pockets, taking a public stance on corporate strategy does not come easily to many of these funds, in part because they are unaccustomed to readying the kind of presentations aimed at swaying other shareholders.

For example, Wellington’s statement on Bristol-Myers Squibb’s Celgene deal was just four sentences long. By contrast, Bristol-Myers published a 46-page document defending its deal.

The world’s biggest active mutual fund managers, including Fidelity Investments and Capital Group, have preferred to use their influence discretely, taking advantage of their access to management to gain insight into a company’s strategy and offer feedback behind closed doors.

To stay on good terms with corporate management, large mutual funds have often been happy letting activist hedge funds agitate over a company’s perceived problems.

To be sure, even passive investors have started to pressure companies behind the scenes, especially on social, governance or climate change issues that a younger generation of investors cares more about.

For example, BlackRock Inc and Vanguard Group voted against management at oil major Exxon Mobil Corp in 2017 over its reluctance to disclose the risks it faced from climate change, and pressured weapons manufacturer Sturm Ruger last year over its refusal to publish a report about the safety of its products.

“Corporate America had better take note because the folks who actually pick stocks have finally decided to flex their muscles,” wrote Don Bilson, head of Event Driven Research at Gordon Haskett Research Advisors.