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Researchers have tracked 89 northern saw-whet owls (Aegolius acadicus) along a migration corridor in Western Montana, underscoring the efficacy of telemetry studies for detailed investigations into the movements of birds.

Their paper, "Migration and Roosting Behavior of Northern Saw-whet Owls (Aegolius acadicus) During Fall Migration in Western Montana," is in the Journal of Raptor Research.

The research team was surprised to discover that owls demonstrated stopover behavior, meaning pauses along migration for resting and refueling, as well as notable individual variation in their direction and method of travel. These findings further unveil the migration story of North America's smallest owl, which may accelerate research on the movements of other owl species as well.

Saw-whet owls are roughly the size of a human hand. They are difficult to detect at roost sites, given their fondness for high perches and the very real possibility of humans confusing them with a pine cone. During migration, they don't leave reliable "whitewash" or pellets below their roost sites either.

"Even when the owls were low in roosts, they could be so cryptic that you could stand within feet and still not see them," says lead author Kate Stone. "I wish I could be that elusive in the woods."

We still don't know to where saw-whets actually migrate.

"They are incredibly common and numerous," says Stone, "so, it's mind-boggling that we know so little."

Traditionally, saw-whet owl migration in the west has been inferred based on data collected from leg bands. Recapturing a banded owl can tell you that an owl is moving between sites, but it can't tell you much about the owl's route, habitat use, or migratory behavior while in transit.

Tracking technology helps fill those gaps. Stone says she is hopeful that learning more about the needs of a common owl may help researchers make stronger management choices for other small owls that are less common or more difficult to study like western screech owls (Megascops kennicottii), flammulated owls (Psiloscops flammeolus), or northern pygmy owls (Glaucidium californicum).

The study occurred during fall 2014 and 2015. Owls were fitted with temporary VHF radio transmitters and then tracked via airplane, with ground confirmation when possible.

Of the 89 tracked owls, the majority behaved like migrants, yet some also demonstrated overwintering behavior. Of the migrants, most were recently fledged females. Nightly distances for the owls averaged between three and eight kilometers (three to five miles), though some owls traveled long distances, including one that maxed out at nearly 66 km (over 40 miles). Not all owls traveled directly south, and over half of the migratory owls tracked for multiple days paused along the way.

"Stopover behavior is really difficult to tease out from banding data, so it's been largely unexplored until now," says Stone. "This behavior shows that saw-whets really are reliant on ground conditions to support their migratory journeys."

Collectively, the results of this study show the complexity of saw-whet owl movements, but also that there are many exciting hypotheses to test in the future. Stone hopes to use longer-lasting tags in the future and automated tracking systems like the Motus network to bolster understanding of owl movements.

"From a conservation perspective, understanding what features saw-whet owls need and the distances they travel could help us make management decisions that support their populations," says Stone.

She adds that this is especially critical for owls that move across landscapes shaped by humans, as many of her tracked owls did. As development increases, migrating birds of all types will face this challenge. The more we know about their routes and needs, the more effectively we can assist them along their journeys.