The outspoken money manager sent an investor alert on Monday noting that the portfolio was off 0.1 percent last month, leaving it up 3.6 percent for the year, according to two people familiar with the numbers.

In the investor alert, Ackman gave no details on what fueled the lackluster performance and a spokeswoman declined to comment.

On the final day of February, Pershing Square was whipsawed by a 17 percent one-day plunge in shares of retailer JC Penney Company Inc (>> J.C. Penney Company, Inc.) and a 7.6 percent gain in shares of supplements company Herbalife Ltd (>> Herbalife Ltd.). For Pershing Square, JC Penny is one of the fund's bigger "long" positions, while the hedge fund has a $1 billion "short" bet that shares of Herbalife will tumble.

For Ackman, one of the $2 trillion hedge fund industry's better-known managers, the year is off to a undistinguished start. The Standard & Poor's 500 index is up about 6.2 percent for the year.

But Ackman isn't the only well-known stock picker to be underperforming. David Einhorn's $8.8 billion Greenlight Capital is also up just 3.6 percent for the year.

Daniel Loeb's Third Point Offshore Fund is up 6 percent this year while his Third Point Ultra fund is up 8.8 percent.

As hedge funds slowly release their latest performance numbers, investors are paying especially close attention to how the industry's biggest managers are faring now that the stock market is moving ahead at a solid clip.

These types of numbers may give investors, especially large ones, more ammunition to press their case for big hedge funds to cut their notoriously high fees, analysts have said.

Still for Ackman's investors, the February numbers offered some solace especially after the fund suffered its big one-two punch on the last day of February.

For Pershing Square and its investors, March however is not getting off to a better start as news reached investors that Vornado Realty Trust (>> Vornado Realty Trust) was offering a block of 10 million Penney shares for sale. Vornado's chairman, Steven Roth, joined Penney's board at the same time Ackman did and speculation has been growing that Roth might be losing patience with Penney's planned turnaround under CEO Ron Johnson. Questions about Penney and Herbalife are lobbed at Ackman with increasing frequency and fervor every time he appears at public events like investing conferences.

While two of Ackman's positions moved against him in February, the concentrated fund, which bets on only a few names at a time, clearly had strong performers to offset the negative news. Investors will not find out for some time what helped inoculate the fund against a bigger fall.

Pershing Square has steadily made itself into an industry darling lining up clients at public pension funds in Massachusetts, New Jersey and Colorado, among others, and plenty of private investors. Recently some investors have privately wondered about how prudent it is for Ackman to have been so public about his short investment in Herbalife where he seemed to have all but tempted other investors like Daniel Loeb and Carl Icahn to take the opposite side of the bet.

(Reporting by Svea Herbst-Bayliss in Boston; editing by Matthew Lewis, Matthew Goldstein and Bernard Orr)

By Svea Herbst-Bayliss