"Free checking is really a major symbol of the gap between banks and credit unions," said. Michael Moebs, economist and CEO for Moebs $ervices, an economic research organization located in Lake Forest, Ill., said. "They're staying the course with it."

Credit unions use free checking to lure members, Jon Jeffreys, managing partner for the Washington-based consulting firm Callahan & Associates, said.

"Credit unions want to grow organically. They want to build that core relationship," he said. "That program is how you get people in the door."

Moebs' own research found that nationally, 59.3% of financial institutions offered free checking in 2015, with 46% of the banks offering the service and 73.6% of the credit unions surveyed offering it.

For its study, Moebs surveyed 2,791 institutions. The Bankrate credit union data originated from a survey conducted between Jan. 13 and Jan. 20, 2016; the bank results were derived from an October 2015 Bankrate survey of bank checking accounts.

Moebs reported that last year, 47.9 % of banks and 79.6% of credit unions offered free checking.

That still represents a big drop compared to a decade ago. In 2005, 78.6% of financial institutions surveyed offered free checking. In that survey, 71.5% of banks offered the service compared to 85.9% of credit unions.

"Banks have moved dramatically away from free checking," Moebs said, adding that community banks – as well as credit unions – have been more reluctant to abandon the service.

Credit unions have been at the forefront of free checking services for a while, McBride said.

"Free checking was a staple among credit unions long before it was fashionable," McBride added.

While they've been quick to offer free checking, credit unions were more reluctant to pay interest on their checking accounts. According to the Bankrate survey, only 38% of credit union checking accounts paid interest, with the most common yield being 0.05%.

Not one of the credit unions surveyed required more than $100 to open an account and about two thirds did not have a minimum balance requirement.

Bankrate reported the average credit union overdraft fee is $27, less than the banks' fee of $33.

Moebs said his research shows an even slimmer difference between overdraft fees at banks and overdraft fees at credit unions.

"There's no statistical difference," he said.

He also said the revenue generated by overdraft fees is increasing among credit unions.

However, Moebs said members who otherwise might overdraft their accounts – providing the credit union with a source of income – are now going to payday lenders instead sometimes. That cuts into the amount of revenue credit unions generate through overdraft fees.

At the same time, critics have said credit unions should not be so dependent on overdraft fees as a source of revenue. Interchange fees can also provide additional revenue for credit unions, Moebs noted.

Moebs also said credit unions are incurring higher costs because the number of transactions per account holder has increased. He added in the past 15 to 20 years, the number of monthly transactions per account has doubled or tripled. A typical account holder may have generated between 20 and 30 items in the past, he said; now, that same account holder is likely to complete between 40 and 70 transactions every month.

When it comes to ATM fees, 34% of credit unions do not charge a fee for going out of network or allow at least one free out-of-network withdrawal each week, according to Bankrate.

Non-members get hit when they use a credit union's ATM, Bankrate said, with 96% charging for that service. The most common fee is $3 – the same reported at banks.

At the same time, financial institutions, including credit unions, are under pressure from the CFPB to offer no overdraft alternatives to consumers. The bureau recently sent a letter to the 25 largest retail banks, calling on them to offer lower-risk accounts to more consumers in an effort to help prevent overdrafts.

"All of the sources of fee income are under attack," Jeffreys said.

McBride agreed, stating, "Credit unions have been affected by the same regulatory pressures as banks."

Aside from increasing overdraft fees, Moebs said to offset the cost of services such as free checking, credit unions can generate revenue by improving their cross-selling of products to account holders. By looking at households, a credit union might be able to gauge the overall profitability of trying to sell additional services to people at a particular address.

"If you're going to do this, you have to be very mindful of getting revenue," he said.

Moebs said banks are conceding the free checking field to credit unions based on an assumption that financial institutions do not make money on checking accounts.

McBride said he does not believe the percentage of banks offering free checking will drop much more.

"Banks have shown signs of bottoming out," he said.

And he said he does not think credit unions will cut their free checking accounts in order to cut costs. He said the percentage of credit unions offering free checking accounts hasn't moved much since the financial crisis, and predicted the percentage of credit unions continuing to offer the service will not dip below 72% or rise above 78%.