NEW YORK (AdAge.com) — This summer, agency holding companies kept busy with a bit of housekeeping. Omnicom Group, WPP, Interpublic Group of Cos. and MDC Partners in the past few months have all streamlined the number of individual shops and agency offices sitting under the same umbrella — sweeping underperforming entities under their stronger siblings and combining agencies to create scale and expand geographic footprint. And it's not over: CEOs of those companies and observers say there's more to come this year.