Log Cabin Chronicles

Listen to the night sky

Beth Girdler

BETH GIRDLER

It is getting colder by the day. I even saw a few snowflakes this morning. Although late in the season, I urge you to go outside during the last of the fall nights to look and listen. If the conditions are just right, you will be surprised at what you see and hear.

Quite a few years ago now (I won't tell you how many) in my first year at Trent University I took astronomy. Astronomy was considered a "bird course," an easy course. One cool clear evening in October the term "bird course" took on a new meaning.

The whole class was up on the roof of the chemistry building. We were armed with telescopes and cameras to take photos of Saturn, which happened to be very visible at that time. While gazing at the cosmos, I became aware of movement between myself and the stars. Tiny dark shapes were hurtling past above our heads. At first, I thought I was seeing bats, but the movement was too direct. Then I heard the occasional short chirp and the penny dropped.

These were songbirds heading south. I was informed that many species migrate at night. Conditions must have been perfect that night because the stream of birds was continuous, many passing so close to our heads that I could almost make out which were warblers by their size and shape.

Fall migration allows birds to move to areas with optimal winter food supplies. Some birds travel great distances, moving from the Northern Hemisphere to the Southern Hemisphere and others simply move from one region to the next.

There are several methods of migration. One is for the bird to move south with regular stopovers for feeding and resting up. Each stop may last a day or two. Birds travelling this way do not need to store as much extra body fat as birds that fly nonstop for many hours.

Nonstop migrants may fly for 80 or 90 hours at a time and sometimes much more, covering distances of 500 to over 1000 miles at a go. These birds often double their body weight before beginning their journey. Our tiny ruby-throated hummingbird, gone with the first cold weather, has to cross the Gulf of Mexico twice a year, a distance of 1000 kilometres, burning up much of its reserve fat in the process.

Most birds fly between 1400 and 3000 feet above the ground when migrating. Many thousands are killed every year when they fly into the windows of tall city buildings at night. I came across figures indicating that migration is so exhausting and fraught with peril that only half the numbers of the small songbirds survive the trip.

Approximately 100 songbird species travel at night. Night travel makes sense when you realize that at this time, turbulence from solar heating and attacks by predators are at a minimum. Given a good tail wind and stars for navigation, a bird can cover hundreds of miles each evening.

Many of these night-migrating birds give regular, short calls while they fly. These calls most likely help individuals establish and maintain their location in relation to other migrating birds.

Each species has a distinct call.

Thrushes, for instance, make a short whistled note, several warblers give a short high "tseep," while others give a buzzy "zeet."

Scientists have mapped out which call belongs to which species for many of the night-migrating birds. Using rooftop microphones, the calls of these birds are recorded at stations across North America. A computer program is then used to separate the calls from insect and other sounds, matching them with known calls. This technology makes it possible to record the location, number, and species passing by a study area during migration.

Though still improving, acoustic identification and counting of birds beats the older method of using fine netting or mist nets to trap the birds as they fly through. On a good night, thousands of calls can be recorded at one station.

Migrating birds also show up on radar. Valuable information on numbers, velocity and patterns of bird migration over extensive areas has been gathered using radar observations.

Go out at night (with a pair of binoculars if you have them) and look at the moon. Although most of the smaller birds moved on weeks ago, you may still be able to see a few as they pass in front of the moon on their way south. Have a listen too. If you hear a cricket-like chirp coming from above, know that one small bird, flapping furiously, is making its own incredible journey.

If you don't catch a night viewing this fall, remember to listen next year.

The naturalist Beth Girdler writes from Ayer's Cliff, Quebec.

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