It was a chilly August night on Cameron Pass in Colorado, the most famous site in the southern Rockies to find the elusive Boreal Owl, and Andrew and I were at it again.  Neither of us had lived in Colorado much more than a year, but already we’d made several nocturnal pilgrimages to Cameron, only to stand on the side of the frigid road playing the Boreal Owl tape over and over into the dark, answered by the sound of wind in the trees — or, even more frustrating, the kind of pin-drop silence that might carry an owl call for half a mile or more, if only the owl would call.

Pessimistic but stoic, we crawled out of the car and hit the tape.  The haunting sound went out into the night, over and over again.  And the night answered back with…

SKIEW!

It was an incredibly loud, squeaky bark, like nothing we’d ever heard before — more like a Hollywood ray gun sound effect than anything a bird should say.  We weren’t even sure it was a bird until it started moving from tree to tree, circling the parking lot unseen in response to the tape.

SKIEW!

For fifteen minutes we searched in vain for the source of the Star Wars noise.  Was it a Boreal Owl?  We left that night unsure.

A couple of months later we played the Boreal tape again, this time on a remote road near Summitville in southern Colorado, and again heard the aggressive SKIEW! from the dark.  We managed only a brief and terrible look at the creature in the flashlight beam, but we were finally convinced that we had just encountered our second Boreal Owl.

But were we correct?  The closely related Northern Saw-whet Owl lives in some of the same forests as the Boreal, and according to the BNA account, makes a “ksew” sound that strongly resembles the “skiew” of Boreal — and in fact, Saw-whets have been reported to give their “ksew” sounds in response to playback of the Boreal Owl’s primary song.  (One study got responses to Boreal Owl tapes from three Northern Saw-whet Owls,  seven Great Horned Owls, nine Barred Owls, and one Northern Pygmy-Owl — not to mention a few dozen Boreals!)

Can Boreal Owls be identified by their calls in response to playback, or must you get a look at one to be sure?  According to Phil Mattocks (1988),

The beginner should beware that there are lots of things that go “skiew” in the night. However, none of them sounds exactly like a Boreal Owl, according to those in the know (Bart Whelton, Dick Cannings).

In the hopes of joining those “in the know,” I tracked down an internet gallery of known and possible Boreal Owl “skiew” calls and Northern Saw-whet “ksews”:

1. Harry Lehto recorded some nice “skiew” calls from Tengmalm’s Owl (as it is known in Europe) in Finland on 19 October 2008.  Although there are some slight differences in vocalizations between the European “Tengmalm’s” and the North American “Boreal” Owl, the two are still considered a single species, and of all the recordings I’ve heard, Harry’s are the best match for the “SKIEW” I’ve heard twice now in Colorado.
2. Martyn Stewart has posted some Boreal “skiew” calls from the slopes of Mount Rainier, Washington state, from October 2009.  Martyn wrote to me that he didn’t get a visual on these birds, but that they are certainly Boreal Owls.  Note the two different birds giving two different versions of the “skiew,” one much briefer (sharper) than the other.
3. My recording of an unknown “skiew” — very possibly a Boreal Owl — from Rio Grande County, Colorado, in October 2007, in response to playback of Boreal song.  It’s not quite as high-pitched or sharp as the loud calls I was hoping to record, but it’s a plausible candidate for Boreal.
4. The audio gallery of the  Birds of North America account of Northern Saw-whet Owl (subscription required) includes a nice recording of the “ksew” call, plus another recording labeled as a “winter” call, which appears to be the same thing.  The “ksew” calls are very similar to the longer version of the “skiew” on Martyn’s recording, and they’re not too far off from the calls in my recording.

According to Dale Stahlecker, the “skiew” call of Saw-whets is “noticeably weaker” than that of Boreals, and I’ve been making the basic assumption that anything as explosive as the “SKIEW” I heard on Cameron Pass has to be a Boreal.  But as the recordings above show, there may be overlap in the “weaker” versions of the call.   Even Bondrup-Nielsen’s original spectrogram of the Boreal “skiew” is too low-pitched and shallowly inflected to match what I’ve heard in Colorado.  Same goes for the recording of the Boreal “skiew” on Cornell’s Voices of North American Owls collection.

When it comes right down to it, how do we know what’s going “skiew” in the dark?  And if we don’t know, how are we going to find out?