The great irony of the mortgage market can be found in the MBS holdings of depositories: Even though banks are ceding origination market share to nondepositories, they continue to gobble up bonds backed by home mortgages. As Inside MBS & ABS noted last month, bank holdings of residential MBS hit a record $1.643 trillion at yearend 2015, a 2.2 percent sequential gain. Of course, a large chunk of that gain can be explained by Bank of America increasing its MBS holdings by a hefty $36.7 billion during the fourth quarter. The big question for banks – as well as real estate investment trusts – is...