Song use in duetting Central American wrens

In the ten years before my retirement as Professor of Natural History at the University of St Andrews in 2007 I spent much of my research time studying duetting in birds. This is a fascinating yet perplexing phenomenon, found in many bird species but largely restricted to the tropics. Although its exact form varies considerably between species, in all cases the male and female of a mated pair combine to produce a joint song, often with no overlap between their contributions and such tight coordination that the uninitiated observer could easily imagine there was only one bird singing. The major concentration of my research, funded by a grant from the Trust, was on wrens of the genus Thryothorus in Latin America. My Emeritus Fellowship is enabling me to follow up a few of the species in this group which are particularly interesting; I am doing this in collaboration with my former research assistant Dr Nigel Mann, now at the State University of New York, Oneonta, and Dr Christopher Templeton, who is visiting me on an International Fellowship from NSF.

A major finding of our earlier work was that the 28 or so species of Thryothorus wrens were not a closely related group at all, but should be split into four different genera, with one species retaining that generic name and the others now relabelled as Pheugopedius, Thryophilus and Cantorchilus. The typical singing pattern is rather different between these new genera, but some species also differ from the norm for their genus and we are interested in finding out what has led this to be the case. As the birds are also often very difficult to observe in the dense tropical forests they inhabit, our first two field trips have also involved the development of new methods for studying them.

A happy wren in the hand JPG
A happy wren in the hand.

In May and June 2011 we spent three weeks at the Chamela field station of the Universidad Nacional Autónoma de México , which is in dry forest close to the Pacific coast. Here we studied the happy wren (Pheugopedius felix). We carried out an experiment exploring the reactions of a female to intruders in the absence of her mate, which involved taking the male into captivity late in the evening and releasing him after testing her with songs played back from loudspeakers in the morning. We also used the male himself, while in captivity, to examine how these birds coordinate their songs. Each bird has a repertoire of 30 or more song phrases but the members of a pair sing particular ones in combination.

Dry forest at Chamela JPG
Dry forest at Chamela.

We tested each male, held in a small cage covered with translucent material in which he settled well, with a series of phrases of one type recorded from his female. To our delight and surprise this worked remarkably well, most birds responding to the majority of playback songs. The male would usually respond with the appropriate one from his repertoire, and in several cases he did so within half a second on the very first playback: quite a feat given the processing involved. We were able to make a number of other discoveries using this new method and it shows great potential for the future given the difficulty of studying the coordination of duetting in free-moving wild birds in dense vegetation and the much greater control that playback to caged birds gives one.

A pair of plain tailed wrens in the hand JPG
A pair of plain-tailed wrens in the hand.

My second field trip was in November 2011 and was to study plain-tailed wrens (Pheugopedius euophrys), in the bamboo forest at around 3500m on the slopes of Pasochoa volcano in Ecuador. These birds have a particularly complicated duet, with repeated cycles of phrases in the form ABCD, with the male singing A and C and the female B and D. Not only that, but they live in groups and all the birds join in to sing in synchrony. Though each bird has a repertoire of phrases, all those of one sex sing the same one at the same time. It is likely that these groups consist of a breeding pair accompanied by young of previous broods staying to help their parents rear subsequent offspring.

Bamboo forest on the slopes of Pasochoa volcano jpg
Bamboo forest on the slopes of Pasochoa volcano.

To see whether this is correct, we collected small blood samples from some 50 birds and hope that DNA analysis will tell us about their relatedness. But we are also keen to discover why these birds sing in such tightly coordinated choruses, unlike most species in their group. Could it be that a group song is more intimidating to other birds so that it works better at defending the territory? To test this idea we did a playback experiment in which groups were tested with a simulation of two birds duetting or of four singing in a chorus. We will see whether these led to different responses and also whether the response depended on the number of birds in the group to which the songs were being played. After all, playback of four birds singing should be more intimidating to a group of two than it is to a group of six. Whether there really is “strength in numbers” remains to be seen when we have analysed the results.

Understanding how and why these birds communicate in the complex way that they do presents quite a challenge. Our earlier comparative study of this whole group of wrens laid the foundations for a great deal of further work. The studies I am carrying out during my Emeritus Fellowship will hopefully help us to understand how they coordinate their singing and why these very complex forms of singing are beneficial to them.

Professor Peter Slater

Peter was awarded an Emeritus Fellowship in 2010.