Bird Ruminations

I'm sitting in my outdoor tub as twilight approaches, listening to the unfolding of the bird sounds and activities, as dusk begins to come over the land. There is such a variable chorus of birdsong all through the day—from the first pre-dawn hesitant outbursts, to the glorious morning ensemble of messages, to the midday business announcements of daily bird goings-on, to the late afternoon showy serenades, to the evening sign-offs that precede their settling in for the night.

As dusk approaches the calls become more like individual solos, as some of the birds retire, while others can't resist a few last pronouncements. After awhile, I find myself listening to what at first seems to be the last two birds calling out, into the approaching gloom. As I tune into their songs, I begin to realize that they are far more in sync with each other than I first thought. How do they coordinate their songs so well? But I soon come to understand that what seemed to be two birds singing from two nearby locations is really one bird, with two distinct calls.

Then I realize that what is really going on is that this single bird has both a lower-pitched song that I can associate with its true location, and a second, higher-pitched song that is being scattered back to me from the leaves of trees up in the woods. My acoustics training helps me to understand that the lower frequency song travels through the woods, unmindful of the trees leaves, so I can discern the bird's true position: over there in the clearing. But the higher-pitched part of his song has much shorter wavelengths—wavelengths that I realize are about the size of the leaves, so these shorter waves “see” the leaves as much bigger than the low-frequency waves. They reflect off the leaves, back to me, making me think that the bird is somewhere up in the woods where the trees are, rather than over in the clearing.

It's similar to when you drive at night with your headlights on. On a clear night your beams penetrate into the distance, illuminating the road ahead. You can see far off, knowing where the road ahead is, and see the distant oncoming cars by their headlights. On a foggy night, however, the air is saturated with water droplets whose size is “seen” by the waves of light from your car's beams—just like the tree leaves for the high-frequency birdsong. The result: your headlights' glow is reflected back to you, dramatically altering your view. Your eyes are dazzled by your own headlight reflection and you lose perspective of where you really are, or even where those approaching cars are.

This phenomenon (for birdsong or headlights) is called “backscatter.” It can disorientate you—even make you think there are two birds out there, rather than one late caller who is about to retire for the evening. A closer listen helped me tune in to see what's really happening. “One bird in the ear is worth two in the tree.”