A few weeks ago I wrote about the nocturnal migration of songbirds. A few people responded so enthusiastically that I thought I’d write a little more in depth about it.

I happened to be up very late one night earlier this week. I had opened a window to get some fresh air, and heard a lone coyote howling in the distance. Then, the call notes of birds came to my ears. I started counting the number of calls I heard per minute; the highest I came up with was 21 calls in a minute. There was a major migratory movement going on right overhead.

It was hard to tell how high up the birds were, but some of them sounded as if they were passing just over tree level. Others sounded as if they were coming from quite a bit higher. In “How Birds Migrate,” author Paul Kerlinger states that several studies on this subject have shown songbird migration takes place within 2,000 feet above the surface of the earth.

Altitude can vary depending on wind conditions and temperature, however. Winds at high altitudes are stronger than those nearer the surface of the earth, so if a bird is flying into the wind, it will fly at a lower altitude. Conversely, if there are good tailwinds, the birds will fly higher.

Temperatures are also cooler at higher altitudes. This is important for birds using powered flight (as opposed to gliding or soaring flight), as they need to dissipate the heat generated by the energy they use.

I wasn’t expert enough to identify the makers of the calls, although some birders can. Many of the notes were sharp, loud “seets,” while others were drier, more metallic-sounding “chips.” Why do these birds make any sounds at all? Scientists speculate they do it to keep in touch with one another, either because of poor visibility or because they are so numerous the calls serve to prevent in-air collisions.

As far as navigation goes, it may seem obvious that birds would use the stars to help them find their way. And so they do, but they also use the sun and its plane of polarized light; the moon; the wind; the topography of the earth; olfactory cues; and, most importantly, the earth’s magnetic field.

It’s a fascinating subject, one scientists continue to ponder and study. I’m just glad I’ve learned to appreciate the phenomenon, and to enjoy a starlit night filled with migrating bird calls and the howls of a coyote.

BDN bird columnist Chris Corio can be reached at bdnsports@bangordailynews.net