Rules to Sing By
Unraveling the mysteries of bird duets in Puerto Rico's mountainous rainforests.
“Backlighting is a big problem,” David says about the general problem of finding birds. There is also the fact that most of bird life––the singing, the flight, the sparring, the mating––goes on over our heads.
“Try calling them,” he prompts, and Daniel holds the digital recorder away from his body to broadcast the warbler's song.
But if there are elfin-wood warblers here other than the one that passed us on its way up the mountain, they aren’t paying any attention to the songs Daniel plays back. At about 10 a.m., we are working our way down the mountainside, and the few birds we have been hearing seem to tire of the singing effort.
Across most of North America and in other temperate climates, birdsong is performed almost exclusively by male birds. The male is the one who has the time and the assignment. His mate of the season is busy laying eggs, brooding, and stuffing the oversize maws of their chicks. Although the male may help with the feeding, his main job is to stand guard over the pair's summer territory, and he sings to ward off any intruders, birds of any kind and predators. Their territory, their pairing, and their family are temporary. The birds initiate all this when they fly north and arrive in their nesting grounds, and they end it when the birds scatter back south. “But here in the Neotropics,” David tells me, “bird families are more stable.” They aren’t nomadic, and for many species, the pairs are more or less permanent. They are monogamous, at least for the social enterprise of raising a family. Because tropical birds aren’t working under time pressure to produce chicks that can travel before their impending migration, the breeding and brooding schedules are more relaxed. The male and female share the work of raising the young, and both the male and female birds sing.
Back at the university we sit at David's computer to listen to the duets of the bird we glimpsed so briefly. The thin, rattle we heard on the mountain is there, and it is seamlessly linked to a brief series of sharp chits. The song sounds as if it comes from one bird.
“It may take some time before we get this in the field.” He says and then clicks on a sound file for a duet he understands thoroughly, the songs of the black-bellied wren. The wren is very shy and elusive, and compared to the truly flamboyant birds in the Canal Zone—the toucan, the flashy parrots, and the bright green but stolid motmot—the black-bellied wren’s coloring is rather dull. It is the size of a large sparrow, and for a little decoration, it has a stippled black pattern below its white bib. Although a whole family of these wrens will often move about together, their coloring and their secretive habits enable even a group of four or five of them to stay out of sight.
What the bird lacks in flash it more than makes up for in song. Both males and females are prodigious singers. They sing in distinct phrases and repeat them. And repeat them. They seem to find it hard to stop. It's endurance singing.
David imitates the typical phrases, making a low little scat song and tying it up at the end with a rising phrase. "Whee-pheew, foh-loddle-loddle-lay. The guidebooks say it sounds like cream-of-wheat. That’s not what I get out of it. But all you have to do is hear it, and you’ll know it.”
True. The male and female wrens are committed to their continual enterprise of song. The male bird sings, the female responds––over and over, without an audible pause between each bird’s part. There is no slice of silence. You have to have very good ears to recognize that there are two singers.
“I think I'm the first person to look at the structure of duets," David says. "Each male bird has probably forty different songs he knows how to sing.” In the bird world, this is a good-sized repertoire but not so prodigious as the number of tunes learned by some of the song sparrows. “But listen, . . .” He re-clicks the same sound file, and says, “A pattern, right? It’s a little different, but there’s still a pattern.”
After he plays a couple of long bouts of these exchanges, he asks “So why do two birds do this duetting thing? How does it help them succeed?” A recent scientific review of duet singers makes clear that the function of a duet probably varies from one species from one species to another. But there are some constants.
Singing is costly. It consumes a bird’s energy and time and it alerts predators to the bird’s presence. But it doesn’t cost the bird as much as actual combat with competitors, and because sound is a very rapid, very effective means of communication, singing is a good substitute for physical contact. Singing or calling can divert an approaching bird intruder or competitor or ward off a predator before an attack becomes necessary. It can mediate disputes and spares the birds and danger and energy of physical assault. It has also been proposed that the strength of singing also helps to attract and guard the female because female birds may equate the amount of energy a male bird puts into singing with the amount of energy he will put into helping her raise their young.
"Most of the work on duets looks at what they do, what function they serve," David explains, "and one of the dominant theories is that they are cooperative territory defense. There’s also been some speculation that this continual singing is a way of staying in touch to prevent either bird from flitting off for a quick mating with another bird. But that’s not my take on it."
He brings up the volume on the wrens' duet. “Listen to how hard she’s working to get it right." There is an anxious obsessive quality to the female’s timing of her entries.
“Why is it so important that she get it right?” David asks another rhetorical question.