Boreal chickadees look similar to black capped, except that their cap is brown instead of black, which makes them a bit difficult to spot especially when they mix in with black caps.
Northern Minnesota is one of the few places in the continental US that you might have a chance to see these birds. They typically spend all of their time up in the boreal forests of Canada and Alaska but during the winter, when their is less food available, they will cross into the most northern reaches of the US, which includes northern Minnesota. These pictures where all taken up in the Sax Zim Bog area back in January (1-10)
The problem is that a big part of their diet comes from insects, which are not very abundant during the winter time up in Canada and Alaska. They compensate for this lack of food during the winter by cache food during the warmer months. They supplement this cached food with pine cone seeds but in year where the pines produce less cones those birds that do not have enough food cached will head south in search of sustenance.