One trend prior to 2011 was that large, publicly traded firms initiated about one-third of the U.S. deals, Morrissey says. But in 2011 and 2012 only about one-fifth of such firms did so "because a lot of them turned their sights on overseas markets where they saw relatively greater opportunities."

As for major acquisition targets, firms with expertise in energy, particularly natural gas, and/or environmental sustainability are particularly attractive now, he adds.

Recent deals that appear to have capitalized on those trends are Albany-based CHA Consulting's purchase of Boston-based Coler & Colantonio, a 175-person firm whose services include energy infrastructure, land development, environmental, and water and wastewater design and operation. Also, New York-based Thornton Tomasetti acquired San Francisco-based Simon & Associates, a six-person firm that will become part of Thornton Tomasetti's Building Sustainability practice.