Complex Bird Calls

Sitting in the outdoor tub with the stillness of night stealing in, I love to listen to the evening bird calls, as each species takes its turn, before flying off to the night's roost. I can almost predict the sequence of calls, as the titmice and goldfinches begin the evening's sign-off, followed by a tumultuous Carolina wren. Then the wood thrush and the cardinal vie to see which one closes off the day's symphony, before the whippoorwill ushers in the night chorus.

I love to listen carefully to the various calls, trying to discern if birds of different species seem inclined to listen to each other—if only to determine when their own song can be offered by slipping it in between the calls of others, so as to minimize the overlap and maximize the chance that each of them will be heard. The intent of their singing is not to compete with a different species, but to signal any nearby birds of its own species that it's here and deserves respect. (In a similar fashion, it has been shown that birds singing in cities modify their calls, in an attempt to be heard above the din.)

Most of the calls I recognize come from old friends, with whom we've cohabited in this clearing in the woods for a few decades now. I have come to expect their voices each evening. But now and then I hear an unfamiliar call and wonder who it is. Might it be a visitor who's testing whether it might find a home here? Might it just be passing through and stopping on a pleasant evening, to join the chorus?

This particular evening brings a new, rather complex call. It seems to me that it's two different birds—one shouting out a high-pitched melodious squawk, sort of like a soprano bluebird. The second call is a lower-pitched “burp, burp, burp.” It must be two different species, I think, with such contrasting calls. They even seem to be coming from two different directions in the woods.

But as I listen over several minutes, I begin to become suspicious about my first impression. These two different calls are precisely synchronized. They quickly follow one another in the same succession, but never overlap. When I normally listen to two different species of birds calling out, they seem to be trying to fit their calls into each other's gaps, but inevitably get a little sloppy and overlap each other just a bit. These two calls tonight never overlapped.

So maybe I'm listening to just one bird that sings out with two very different calls—one a high-pitched squeal and the other a low-pitched burp? That could be. The wood thrush has a three-part call—each part having a very different quality. And I've written here before how the different parts of the wood thrush's call even seem to emanate from different locations in the woods. I have decided that the wood thrush's song appears to come from different directions, because one part of the call reflects off trees leaves, while the other part penetrates deep into the woods.

So what am I listening to this evening—two birds who have an uncanny ability to perfectly synchronize their calls, or a single bird with a two-part call? I do seem to hear the call coming from two different locations in the woods. The answer will not come tonight, as the calls soon ceased. The bird (or birds) will hold onto their secret for another day.