Pershing Square Capital Management CEO Bill Ackman

Pershing Square Capital Management CEO Bill Ackman

Peter Foley/Bloomberg

Bill Ackman’s activist hedge fund, which has partnered with serial acquirer Valeant Pharmaceuticals International Inc. to offer to buy Allergan and reap cost savings from combining the rival companies’ skin- and eye-care businesses, began buying Allergan stock Feb. 25 and then in March switched to over-the-counter call options to accumulate his stake, regulatory filings show.