No matter how many times I witness the spectacle of bird migration, it never ceases to awe and humble me. And the most magical experiences always seem to occur in the late hours of bright, moonlit nights, when the air is completely still and everything is silent.

Everything, that is, except the birds passing by, unseen, overhead.

It all started when I opened my e-mail to see a post on the Maine-birding list about the nocturnal migration under way. It had been sent earlier that evening, around 8:45 p.m., but from the sound of it, there was a big event going on out there. As I stepped out onto my deck, I immediately heard the call notes raining down from above.

Every 10 or 15 seconds or so I heard a range of different flight notes: full, rich zeets to flat, high-pitched chips. I’m not experienced enough to identify a bird by its flight note, but there are those who are, and I envied them at that moment.

As I checked my e-mail again, I saw another birder had posted a link to a Web site showing radar images of the area – the migration was being picked up and tracked by radar. In the past, no one ever knew what caused these mysterious images, and they were whimsically dubbed “angels.”

Now that more is known about bird migration, specifically nocturnal migration, radar is used by ornithologists and amateur birdwatchers alike to track the scope and depth of this phenomenon. As I refreshed the Internet connection, I watched the area around southern Maine glow green, shifting and expanding with each download.

I returned to my deck and now, the flight notes were continuous. From the sounds of it, there was a river of birds flowing by overhead. Researchers think birds vocalize in flight to maintain communication with one another. In “How Birds Migrate,” ornithologist Paul Kerlinger states that birds call more often when visibility is poor or when they are otherwise disoriented.

In addition, he says, they may call more often when confronted with a barrier to migration – such as a coastline. Kerlinger cites studies done at Cape May, N.J., where more birds called within a few hundred feet of the ocean than just a half-mile farther inland. But, he adds, “Whether there are more migrants close to the coast or whether they simply call more is not known.”

The number of calls I was hearing that night seemed to indicate many migrants were passing overhead. I wondered if the ocean, which is a half-block away from me, was concentrating the birds into a narrow band that followed the coastline. Small songbirds will avoid flying over sea if they can at all help it; they need to be able to put down to rest, refuel or escape bad weather.

Birds that are blown out over the ocean have a poor chance of reaching land again, should they tire. Sadly, one report on the Maine Birding mail list told of a flock of songbirds that had been blown off course. The exhausted birds attempted to land on a research ship to rest; many did not make it and just fell into the sea.

Faced with such overwhelming odds to their survival, it makes sense birds would try to maximize their chances. Many, such as the small-to-medium-sized songbirds, do so by migrating at night.

The night air, in general, is much calmer and smoother, making it easy for birds that must rely primarily on powered flight to stay aloft. Large birds – such as hawks and eagles, for example – must expend too much energy to rely on powered flight alone. They migrate during the day, when the air is more turbulent and thermals – rising columns of warm air – are rising high into the sky.

This provides lift to the birds, enabling them to gain altitude with a minimum of energy. Once they reach the top of the thermal – which is starting to dissipate – they set their wings and are able to glide for a long distance without needing to squander energy by flapping.

The night air is also cooler; this is important, for birds generate a tremendous amount of heat to maintain powered flight.

As I listened to the flow of birds above me, I wondered at the daunting challenges they faced, and the miracle that so many of them make it to their destinations.

bdnsports@bangordailynews.net