Tree Swallow at one of our Bluebird houses 4 years ago at this same time.  We saw quite a few Tree Swallows last week.  Mid- April of 2018, 2019, and now, 2019 have all been record and near record cold in S.E. Minnesota (not true in most of America).  This has resulted, and probably will again, in the Tree Swallows taking a big hit, as they are insect eaters. We have not had a nesting Tree Swallow at our place since 2017.  I hate posting bad news but the small bird population in North America is in rapid decline, mostly due to habitat lose.