Birder's World Field of View: Birdsong expert and Ivory-bill searcher Dan Mennill discusses Costa Rica’s duelling wrens

Ornithologist Dan Mennill holds a Rufous-and-white Wren, a species he studies in Costa Rica. Photo by Dale Morris.

Today, our good friend Dan Mennill, an ornithologist at the University of Windsor, is publishing a paper with Sandra Vehrencamp, a scholar of animal communication and professor at Cornell University, about the unusual duetting behaviors of a bird from Costa Rica, the Rufous-and-white Wren. Dan first came to our attention two years ago this month when he and fellow scientist Geoff Hill reported finding Ivory-billed Woodpeckers in Florida. (Hill wrote about their search in our February 2007 issue in the article “The Other Guys.”) This week, we checked in with Dan about his new paper, about what it’s like to work in Costa Rica, and about the Ivory-bill. His answers appear below. — M.M.

Where do you and your students do field work in Costa Rica? What was it like to work there?

We work within Santa Rosa National Park, in the Guanacaste Conservation Area of Costa Rica. I have been working in this park for six years, together with my wife, Stephanie Doucet. (Stephanie is also an ornithologist at the University of Windsor, and is an expert on feather coloration in birds.) The forest is a mature neotropical humid forest, surrounded by patches of neotropical dry forest. Four-hundred-year-old Guapinol trees form the forest canopy; flocks of parrots fly overhead; troupes of howler monkeys start each day with a roar; and Long-tailed Manakins perform their acrobatic dances everywhere you look. It is an extraordinary forest, and it should be on every birdwatcher's to-do list!

The Rufous-and-white Wrens spend most of their time moving secretively through the thickest undergrowth in this forest. I've worked with a lot of different species of birds, including chickadees, sparrows, warblers, Australian whipbirds, and South American pihas. But these birds are some of the most difficult birds to observe that I’ve ever encountered. Following a Rufous-and-white Wren for more than a minute or two is nearly impossible, given the thick vegetation. I think this behavior of moving secretively through the thick tropical forest is one of the reasons that these birds have evolved complex duetting behavior.

On a typical day in Santa Rosa, we wake up and hike into the forest in the pitch black. My research assistants and I will have set up the eight-channel Acoustic Location System the previous afternoon, so all we need to do in the pre-dawn hours is carry the laptop computer to the designated spot, and plug in the eight microphones. Around 5 a.m. the first light of day comes over the volcanos, the Rufous-and-white Wrens begin to sing, and I begin to record their songs with the eight microphones. I let the system run for about five hours, to catch the wrens singing during the most vocal part of the morning. At the end of the recording session, as the weather is getting very hot and the birds are becoming quiet, I shut down the computer and take it back to our base camp. There I download the massive sound files. (One morning of recording can generate more than five gigabytes.) At the end of the field season, I take all of the recordings back to my Sound Analysis Laboratory at the University of Windsor. There, I use state-of-the-art software to triangulate the positions of the birds based on the eight-microphone recordings.

A Rufous-and-White Wren's song is flute-like, Mennill says. Photo by Dale Morris.

In your paper in the latest Current Biology, you write about Rufous-and-white Wrens. Why that species (and not any of the other interesting birds in Costa Rica)?

The first time I traveled to Santa Rosa National Park, I was completely blown away by the voice of the Rufous-and-white Wren. This species belong to the Thryothorus group of wrens, which are famous for singing coordinated vocal duets. But Rufous-and-white Wrens stand out because their songs are much lower-pitched -- much more flute-like -- than any of their relatives. They are beautiful, tuneful songs.

I distinctly remember the first duet that I heard from a Rufous-and-white Wren. I had been trying to catch a glimpse of a bird that was singing solo songs from a thicket in front of me. After the bird sang a series of solos, its partner began to sing overtop of his songs, and they carried on to sing a short series of duets. From where I stood, it was easy to distinguish that the songs were coming from two different birds, and not just one.

Duetting is common in about 220 species of birds, most of which live in the tropics. But the function of these coordinated displays has puzzled ornithologists for a long time. The Rufous-and-white Wren turned out to be an excellent species to study, in order to gain insight into the ecology and evolution of vocal duetting. They perform duets often, and both sexes can sing in response to their partner’s song to create a duet. Importantly, their songs work well with our specialized software for triangulating the position of singing birds.

What does your multi-microphone recordings tell us about duets that we did not know before? Could we expect to find the same behavior in other species? Could it happen in North American species as well as tropical birds?

The eight-microphone Acoustic Location System that I used to record Rufous-and-white Wren duets provided me with a tool to understand where birds are when they sing coordinated vocal duets. Up until this point, field ornithologists lacked a suitable technology for finding the locations of birds while they perform duets. The multi-microphone recordings show, for the first time, that these birds perform duets with tremendously variable distances between the male and female. Sometimes they will duet while they are perched on the same branch. At other times, they will perform duets while they are more than a football field apart.

Dan Mennill sets up a field microphone in Costa Rica's Santa Rosa National Park. Photo by Dale Morris.

In addition, I discovered that birds approach each other as they perform duets. For Rufous-and-white Wrens, duetting appears to be a sort of a "Marco Polo" game where one bird sings a song, listens for a response from its partner, and then moves toward its partner after hearing the response. Duets provide them with a method of maintaining acoustic contact when the thick vegetation makes direct contact impossible. So, one of the functions of duets is in acoustic contact.

I also used a second innovative technology: Multi-speaker Playback capable of simulating the voices of both members of a pair of Rufous-and-white Wrens through separate loudspeakers. I used this playback technique to simulate a rival pair of birds intruding into the territory of a resident pair of birds. I found that Rufous-and-white Wrens respond very aggressively to the playback, demonstrating that duets have a second function: acoustic territory defense.

Importantly, this research shows that duets serve multiple functions, and that the function of coordinated vocal duets varies with context. In a passive context, birds use duets to keep tabs on each others' position. In an aggressive context, birds use duets as audio warfare to defend their territory from rival birds.

You’ve been a key member of the team searching for the Ivory-billed Woodpecker in Florida. Are you or your students still active in the search? What’s the latest?

We're presently wrapping up analyses of the data we collected during our third year of searching on the Choctawhatchee River in Florida. Once again, Geoff Hill of Auburn University coordinated the visual search and the camera-based search, and I coordinated the bioacoustic search. This year we recorded more than 6,000 hours of recordings from listening stations on the Choctawhatchee and a dozen other sites throughout the Southeast that will serve as points-of-comparison for our recordings from the Choc.

My ongoing research in Costa Rica has been important for our Florida work. The forests of Santa Rosa National Park are home to a healthy population of Pale-billed Woodpeckers. These birds produce a double-knock display that matches historical descriptions of the Ivory-billed Woodpecker's, and my Costa Rica recordings provide insight into daily patterns of signaling in Campephilus woodpeckers that help us to hone our bioacoustic scanning technique for the recordings from Florida.

More woodpecker results will be coming soon.

Read Geoff Hill's February 2007 article about the search for Ivory-bills in Florida.

See a chronological list of all our Ivory-bill articles.

See Dan Mennill's Ivory-bill website.

See more photos by Dale Morris.