Smaller mergers made up the bulk of deals through midyear in a trend that most observers expect to continue in coming months.

Many larger banks, with a handful of exceptions, have remained on the sidelines so far in 2015, which has opened the door for midsize acquirers, allowing them to snag deals that might have eluded them in previous years.

Overall M&A numbers at June 30 were relatively unchanged from a year earlier, based on data from Keefe, Bruyette & Woods. At 137, the number of deals rose by just five sellers, while the premium to tangible book value fell nominally, to 141%.

Still, investment bankers are confident that they have enough deals lined up to assure a steady flow of activity for the rest of the year.

"Our pipelines and our peers' pipelines are full of small and midsize M&A," said Emmett Daly, a principal at Sandler O'Neill. Potential deals are "being driven by the overwhelming cost of compliance and the uncertain interest rate environment."

While the overall numbers are barely budging, acquisitions this year "are becoming more specific," said Jack Thompson, head of financial institutions investments at Gapstow, adding that buyers are keen on finding ways to enhance their franchise.

The $11 billion-asset Western Alliance Bancorp in Phoenix, for instance, bought the $1.9 billion-asset Bridge Bank in San Jose, Calif., in a deal that allowed Western to enter certain niche businesses and the attractive South Bay area of San Francisco, Thompson said.

"I don't know if these were necessarily deals that were planned for in 2007 and then didn't get done," Thompson said. Buyers now seem intent on building banks that are "interesting and compelling."

Average deal values were noticeably higher than a year earlier — $137 million in 2015 compared to $71 million at mid-2014 — but were juiced by Royal Bank of Canada's $5.3 billion acquisition of City National and PacWest Bancorp's $847 million agreement to buy tech lender Square 1 Financial.

The lion's share of mergers was much smaller. Through June 30, 118 banks, or roughly 85% of the year's sellers, had less than $500 million in assets, based on KBW's data.

The number of small sellers should easily double by yearend, said Lee Burrows, chief executive at Banks Street Partners. Some of those institutions may have trouble growing organically or raising capital, he added.

"There was a long period where bank M&A was a very normal part of the life cycle for smaller banks," Burrows said. In the five years after the financial crisis, the "option for smaller banks to sell was almost nonexistent because there were no acquirers."

Many interested buyers are larger community banks or smaller regionals and most big banks remain on the sidelines. Heartland Financial in Dubuque, Iowa, and Valley National Bancorp in Wayne, N.J., are among the institutions that have had an opportunity to announce deals that might have escaped them in the past, Daly said.

Heartland has announced three deals this year, including its planned purchase of the $648 million-asset Premier Valley Bank in Fresno, Calif. As a result, Heartland seems poised to complete its eighth deal since 2012.

Heartland continues to talk to targets with less than $1 billion of assets, said Lynn Fuller, the $6.6 billion-asset company's chairman and chief executive. Executives at smaller banks feel like they need to sell, partly due to a lack of succession planning and market constraints such as low interest rates.

"It has to do with smaller banks struggling with the cost of compliance, talent, technology and product offerings to be productive," Fuller said. "At the same time, their gross profit margins are going down. It's been a period of struggle."

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