The winter of 2012-2013 was a remarkable one for birders seeking Boreal Owls, one of the most difficult North American birds to see. That winter there was an irruption of these owls into the upper Midwest, probably due to some sort of change in the abundance of Boreal Owl food in their normal range. Birders flocked to the boreal forests around Duluth, Minnesota, and most came away with their lifer Boreal Owl. But many of the owls themselves were stressed and starving, and through the Field Museum's partnership with the Minnesota Department of Natural Resources, we received several dozen salvaged specimens from northeast Minnesota, birds that were either hit by cars or found dead. The Bird Division recently had a day where many of us--staff, students, and volunteers--converged on the prep lab and prepared as many of these owls as we could.
To give an idea of how much more abundant Boreal Owls were during the late winter of 2012-2013 than normal, below is a chart (created through eBird.org), showing a comparison of the frequency of reports from throughout North America over the five winters from 2010-2014. The massive spike in frequency that can be seen in the increase in sightings during late January and early February, 2013.