Nothing gets birders more excited than widespread reports of unusual birds. They jump in their cars, drive hundreds of miles, and hope to spot a "life bird." Sometimes they are successful; sometimes they are not. It's all part of the thrill of birding.
Right now much of the northern United States is being invaded by snowy owls. From Minnesota to the East Coast and as far south as West Virginia, birding hot lines have been humming with reports of dozens of snowy owls. Every day this week I've seen new reports of snowy owls.
So far most reports are coming from the Great Lakes states and the Northeast. Last weekend one even showed up in Bermuda. In 2011 a similar invasion saw most of the reports originate in western states.
Featured prominently in J.K. Rowling's "Harry Potter" books and movies, snowy owls are well known among the general public. Birders, however, have long held these occasional visitors from the north in high regard. It can take decades to check this species off on a life list. I saw my one and only snowy owl in Oklahoma on the prairie in the early 1980s.
Normally snowy owls live on the tundra of northern Canada and Alaska far from most human observers. When lemming populations are high, females lay as many as 13 eggs. When lemming numbers are low, they lay as few as 3 eggs, or they may forego nesting entirely. Regardless of nesting success, however, every three to five years at least some wander south, presumably in search of food.
Snowy owls eat lemmings, voles, and other small rodents, and periodically these small mammal populations crash. When that happens, the owls can either head south to find food or starve. So they wander south until they find new food sources, which can include a variety of birds and small mammals.
Though a primary predator of lemmings and voles, snowy owls may actually help rodent populations. When rodents are plentiful, owls eat them and their droppings fertilize the barren tundra landscape and stimulate the limited plant growth. This in turn provides habitat for the rodents. But when snowfall is limited, rodents become easy prey and this contributes to periodic population crashes. So perhaps the snowy owls themselves help trigger the population crashes that send them in search of better winter food supplies.