By Svea Herbst-Bayliss

NEW YORK (Reuters) - Billionaire hedge fund manager William Ackman, who has long courted publicity with his multibillion-dollar stock investments, took a new tack on Thursday when he shrouded his most recent bets in mystery.

Ackman's $10.9 billion hedge fund Pershing Square Capital Management made two bets in recent months where the firm committed more than 10 percent of its capital, but it has not revealed the names of those investments.

Hours before welcoming clients to the firm's annual dinner in New York on Thursday evening, Ackman sent a 95-page update to shareholders giving limited information on the new investments. Reuters obtained a copy of the update.

The first new investment, where Pershing Square put 4 percent of its capital to work, presented itself late last year and has already earned the firm a roughly 22 percent return on its average cost, the update said.

"We believe the investment is still attractive at current prices," Ackman said, adding it was a "high quality business" with a "best-in-class management team."

Ackman was even more tight lipped about the second investment, saying only that it was made in early 2017 and that the Pershing Square committed 9 percent of its capital.

Traditionally any news of Ackman stepping into a stock sends the stock price up as investors expect the activist investor to start pressuring management to perform better. And he is often happy to speak at length about what investments he likes and why they should perform better.

But some of his recent bets have been duds leaving 2016 and 2015 to become the worst years in Ackman's recent investment career. The biggest drag on returns was the 2015 investment in drug company Valeant Pharmaceuticals International Inc which lost nearly all of its value amid questions about its accounting and business practices.

Valeant cost Pershing Square 19.2 percent in returns last year. Its most recent publicly disclosed bet, Chipotle Mexican Grill Inc which was made public in September, also lost some money last year, the company said.

But 2017 is starting with gains. Mortgage lenders Fannie Mae and Freddie Mac were pushed higher, in part, last year when President Donald Trump's nominee for Treasury Secretary spoke out in favor of privatizing the government sponsored entities.

In the update to shareholders, Ackman also announced some organizational changes only months after he trimmed the firm's ranks to 60 people from 72, in part to save money.

Tim Barefield, the firm's chief operating officer, who has spent the last decade building out the operation, is retiring. Nicholas Botta, chief financial officer, will become president and Michael Gonnella, currently a senior controller, will be promoted to chief financial officer.

Joe Sutton, Pershing Square's chief technology officer, is leaving the firm this month.

Bharath Alamanda and Feroz Qayyum joined the firm this year as investment team analysts.

(Reporting by Svea Herbst-Bayliss; Editing by Jonathan Oatis and David Gregorio)