Hi Tim, as best as anyone can figure, these snowy invasions are most certainly prompted by a food shortage. It's either the fact that their food source, primarily lemmings, are in short supply - or they had a banner breeding year and the competition for food is fierce. Right now, people are speculating that it was a great breeding year in that most of the sightings seem to be immature birds. That's just the current theory now. Interestingly, unlike a couple of years ago, the invasion seems concentrated more around the Great Lakes and northeast - where, in 2011, it was much more widespread throughout the country.