© Bob Duchesne | Courtesy of Rob SpeirsThis northern hawk-owl has been seen regularly in Lincoln, Maine, recently.
Something weird is going on up north. It's a little early for rare owls to be visiting Maine, but here they come. An invasion from Canada is underway.

The star of the show is a northern hawk-owl in Lincoln. It first appeared about two weeks ago in the area between the Lincoln Regional Airport and Penobscot Valley Hospital. Once a rare northern owl finds a place he likes, he tends to stay awhile. This one has.

Snowy owls have been popping up all over southern Maine. They're also early. The owl sighting in Biddeford Pool doesn't surprise me. They've had a habit of visiting the area for years, usually perching offshore on Wood Island. But the other owls have appeared in places where they are not customarily seen. A young male in Kennebunk settled on top of Mt. Agamenticus for a day. It perched on the rail of an observation platform in full view of an existing web cam maintained by the Regional Conservation land trust. The rare owl spent several hours on camera for the world to see. Now that's just lucky.

It's too soon for qualified scientists to offer opinions on why an owl invasion is happening, but nothing prevents unqualified columnists from speculating. I assume it has little to do with weather. No frigid blasts have forced other subarctic birds such as northern shrikes and rough-legged hawks to fly south in unusual numbers. Generally, invasions are triggered by one thing: food scarcity.

Or food abundance. Maine was overrun by snowy owls last year. The entire U.S. saw an amazing influx of owls. One even showed up at Honolulu International Airport, the first snowy owl ever to be found in Hawaii. The reason is that there was an explosion in the lemming population over the owl's home range during the summer of 2012, leading to a big boom in owlets. Because so many fledglings survived, numerous birds were forced to disperse southward in search of winter food. I surmise that the strategy worked and that these owl baby boomers are still crowding the arctic in numbers too big for the winter food supply to support. So here they come again.

Naturally, we're delighted. Snowy owls are one of the most magnificent birds on the planet - tall and white, with piercing yellow eyes. The great horned owl is slightly larger, but the snowy owl is heavier and equally powerful. Like most arctic owls, it hunts in daylight. In the land of the midnight sun, there is little benefit to relying on darkness.

Snowy owls have no use for trees. On their flat breeding grounds, even a rock provides a sufficient view of the surrounding tundra. Typically, when snowy owls are seen in Maine, they occupy very open areas: beaches, islands, airports, and agricultural fields. They have recurred often along the bare mountaintops of Acadia in recent years. Don't expect them around forests.

Expect northern hawk-owls around forests. They prefer boreal regions of the subarctic, perching and nesting in trees, but foraging over the muskeg. They are sparse even on their home range and are encountered much less frequently in Maine than snowy owls. Maine averages about one hawk-owl visit per winter. One individual haunted Presque Isle for most of last winter, but virtually no others appeared in the state during a year that featured a multitude of snowy owls.

The northern hawk-owl is a unique species, the only member of its genus. It has the exceptional hearing of other owls, and it can hear and snatch prey hiding under a foot of snow. It can see a meal a half mile away. But it has the shorter wings, longer tail, and hunting style of perch-and-pounce hawks, swooping between trees, diving on targets. Its summer diet consists primarily of rodents and small birds. When these are less available in winter, it switches to grouse and ptarmigan for its meals. It can take prey up to the size of a snowshoe hare. In some places, its population follows the 10-year boom-and-bust cycle of the hare population. I am pleased to say that the northern hawk-owl is fond of red squirrels and I invite them to visit us often and eat as many as they wish.

Observe raptors carefully in winter. The snowy owl is large and unmistakable. The hawk-owl is crow-sized and mistakable, resembling a Cooper's hawk in appearance and hunting style. There are a few Cooper's hawks around all year. Let us celebrate the holidays and an owlful winter.