The bluebirds are most likely feeding on berries; my guess would be Eurasian buckthorn or perhaps sumac. There aren’t that many bluebirds that overwinter in MN, but there are a few locations that seem to host them each winter. And with the many berries produced this year on many of our trees and shrubs, this year there may well be more than usual. Unfortunately (this is an issue for the robins as well) berries do not provide much protein, and they face starvation if they cannot find a good protein source by March or so. Last spring dead bluebirds were reported across much of the state because their main food source, insects (high in protein), had not emerged yet due to the relatively colder-than-normal temperatures. To battle their low-protein diets of berries during the winter the birds may occasionally take sunflower seeds at feeders; while not preferred by bluebirds, the seeds do have high protein contents.

There are thousands of robins overwintering in southern MN right now. Usually there are only a few flocks numbering no more than a hundred or so in scattered locations any given winter, but this winter robins are pretty much abundant everywhere. The unseasonably warm weather back in early November may have caused some of these birds to lose the hormonal urge to migrate. Another reason may be the surplus of berries from last year; upon seeing this plentiful food source, the robins may have decided to spend the winter feeding instead of expending energy to go further south. And yet another reason for so many may be that the Dakotas seem to be missing some of their overwintering robins, and thus perhaps some of the western birds came east into MN (probably because of the berries).