Saw-whet populations appear to be highly cyclical, tied to the boom and bust of small rodent populations in Canada. This means the annual migration rises and falls, with periodic irruptions like those in 1999 and 2007.
Saw-whets begin moving south out of eastern Canada in early to mid-September, reaching northern Pennsylvania most years in late September, and central and southern Pennsylvania beginning the first week of October. The migration peaks in this region – appropriately enough – around Halloween, and usually ends by early December. A northbound spring migration passes through central Pennsylvania in late February and March.
Hawk researchers simply have to climb a mountain and count the passing hawks, but we can’t directly observe migrating saw-whets, so scientists are forced to infer much about their migration from when and where they are netted. The largest numbers of owls in fall are caught on nights with a moonless sky, calm or light winds, and chilly temperatures. The best conditions for netting are just after the passage of a strong cold front, with lowering temperatures and a windy day that falls to calm after dark. In spring, balmy nights with a gentle south wind are best.
Saw-whet migration is cyclical, and appears to be tied to a four-year cycle in small rodent populations in the boreal forest of eastern Canada. Major irruptions, as such flights are known, occurred in 1995, 1999 and 2007; an expected irruption in 2003 did not materialize, perhaps because of the impact of West Nile virus, which hit many northern raptor species hard that year. Capture totals for the NSCNA project have ranged from highs of 703 owls in 1999 and 904 in 2008, to lows of 270 in 2000 and 201 in 2006.
There is much we don’t yet know about saw-whet owl migration and ecology. Diurnal (daytime) raptor migrants like hawks, eagles and falcons follow the Appalachian ridges in autumn, saving energy by using the lift generated by winds that deflect up from the mountainsides. We don’t yet know if the owls also follow the ridges (not for lift, perhaps, but because they are a forested corridor through farmland and towns); if they simply travel south in a broad front, ignoring topography; or if they use both approaches depending on the weather.
Neither do we know how far individual owls travel, nor what their habitat needs are along the way. The Ned Smith Center’s research program is designed to answer some of these important questions, because knowing more about saw-whet owl migration is crucial for managing and protecting this unique creature for future generations.
UNRAVELING THE SAW-WHET PUZZLE
Saw-whet owls migrate across most of North America, with some birds traveling thousands of miles each year, but it took several centuries for ornithologists to figure that out. Most early scientists considered them to be permanent residents of the North; in the early 1800s, Alexander Wilson referred to the saw-whet as “a general and constant inhabitant of the Middle and Northern States,” and John James Audubon believed it nested as far south as Louisiana.
By the early twentieth century, though, that view began to change. In the autumn of 1903, a fallout of “small owls” landed on a steamer boat crossing Lake Huron, and three years later, in October 1906, an early snowstorm forced huge numbers of exhausted migrants to the water, where they drowned. Among the nearly 1,900 dead birds collected by an ornithologist were 24 saw-whets, which he called “a surprise… Evidently they migrate in considerable numbers.”
In the 1960s, researchers banding songbirds at the Cedar Grove Ornithological Station in Wisconsin discovered that if they left their mist nets open at night in the fall, they could catch a few saw-whets. They were soon catching predictable numbers of the owls, even though they never saw them by day nor heard them at night.
Other banders in the Great Lakes region followed suit, and in 1986, Tom Erdman of Little Suamico Ornithological Station north of Green Bay, Wisconsin began using a tape-recording of the male saw-whet’s advertising “toot” call. The number of saw-whets he caught increased 11-fold, and the modern era of owl migration research began.
Today, the Ned Smith Center helps to coordinate Project Owlnet founded by David Brinker of Maryland DNR, an umbrella organization comprising more than 120 owl banding stations across North America.