With their eerie hoots and night-time calls, owls are birds of mystery in many ways. They are also hard birds to study since most owls are active when people are home in bed.

While other birds of prey are spotted regularly during Christmas Bird Counts and Breeding Bird Surveys, owls are often missed as they are usually sleeping when birders are out looking for species.

Bird Studies Canada (BSC) is now attempting to solve this dilemma by organizing surveys based on owl-time. Intended to monitor long-term trends in owl populations, the surveys are similar in design to the yearly Breeding Bird Surveys, except that they are run at night.

Started this spring in British Columbia and the Yukon, the surveys are based on pilot projects that have been running in several other provinces. Mike Gill, with the Canadian Wildlife Service, organized the Yukon survey.

Although all of the survey results are not back yet, Gill thinks that about ten surveys were run between mid-March and Mid-April. He says that the advantage of having BSC coordinate the survey is that the group has a lot of experience running volunteer programs.

Setting up a standard procedure for the surveys is a critical task. For this one, volunteers make 10 stops along a 15-kilometre route. At each stop observers listen silently for calling owls for two minutes.

Routes must be started no earlier than a half-hour after sunset and must be finished no later than midnight. Owls are most vocal during this period, as the frequency of their calls decreases between midnight and four a.m.

A study in the Yukon-Charley Rivers National Preserve in Alaska found that Boreal Owls there call most frequently about one to two hours after sunset, while Great Horned Owls are most vocal between one and four hours after sunset.

This timing makes spring surveys in the Yukon a bit tricky; when Gill started his route at 9:45 p.m. in the first week of April, it was still light out. Fortunately, since the routes are short, it was still possible to finish the survey before midnight.

People doing the surveys are asked not to play tape recordings of owls to encourage the birds to call. Gill says that even though tape-playback can be effective tool for some species, the practice can create problems when analyzing the data and can also be disruptive for the birds.

"Playback can give owls the wrong impression. They might think that it's signaling that there is an opportunity to mate, or that another owl is invading their territory. Also, it has been shown that silent listening at this time of year is almost as productive as using playback techniques" he says.

Gill says that the success of the survey does not depend on counting every single owl in the area. BSC wants to provide reliable data on owl populations, and whether they are increasing, decreasing or staying stable in number.

Owls make their calls using low-frequency sound waves that carry over long distances. Great Horned Owls and Boreal Owls are most commonly heard in the Yukon.

The loud hooting call of the Great Horned Owl -- known as hoot owls in some places -- can be heard almost year-round in the Yukon. They are most vocal in February and March during the beginning of their breeding season when the females and males call to each other.

The numbers of Great Horned Owls in the Yukon are known to rise and fall with the ten-year cycle of their favourite prey, snowshoe hares; it has been observed that these owls call less frequently when the hare numbers are low.

In the Yukon, Boreal Owls are typically found in older forests, usually ones composed of white spruce. They can be heard from late February through April, though they are usually most vocal between late March and early April.

Dead or injured Boreal Owls are often reported in the fall and late winter in the Yukon, which is taken as a sign that these birds could be on the move then, migrating short distances between wintering and breeding areas. Boreal Owls are known to migrate in other regions, but their movements here are not well known.Some species found in the Yukon are not likely to show up in the surveys because of their particular habits. For example, Northern Hawk-Owls are not very vocal, though they can often be seen perched along roadsides in late winter. Short-eared Owls migrate north to the Yukon in summer and are crepuscular, meaning that they are most active at dawn and dusk.

Great Grey Owls are most common in the southeast Yukon, while Snowy Owls are arctic residents that are occasionally spotted further south. The Northern Saw-whet Owl has been recorded in the Yukon on a few occasions.

BSC describes this survey as a "made-in-Canada" effort, but hopes that the protocol will be adopted throughout North America as the standard for monitoring owl populations. Gill hopes that as the owl survey becomes better known in the Yukon, more people will volunteer to help with it.

"The more routes you have, the better your ability to detect a change in populations," he says.

For more information on the owl surveys, contact Mike Gill at 393-6760. The website for Bird Studies Canada is at www.bsc-eoc.org.