It takes two: The evolution of duets in New World warblers

Ornithologists uncover relationship between duets and migration.

Everybody knows that birds sing. But did you know that some species sing duets? Males and females sometimes combine their songs into a harmonious melody. The Carolina Wrens (Thryothorus ludovicianus), for example, produce a primitive duet. The male sings a song that sounds like tea-kettle, tea-kettle, tea-kettle, and from time to time the female jumps in with a buzzy sound. You can listen to such a duet in the video below.


Pair Bonds

Ornithologists think that birds mainly sing these duets to defend their territories. In addition, duets might also play a role in strengthening pair bonds. The more time birds spend together, the more likely they will evolve the ability to produce duets. The duration of a pair bond can be limited by migration during which partners go their separate ways. So, you would expect that duets are negatively associated with migration. A recent study in the journal The Auk tested this prediction for the bird family Parulidae (the New World warblers).


A singing Common Yellowthroat (Geothlypis trichas). However, this species does not duet. © Wolfgang Wander | Wikimedia



Liam Mitchell and his colleagues investigated 107 species of warblers and found evidence for duets in 19 species. When they correlated this behavior with migratory strategies, they uncovered a significant relationship. In line with the prediction outlined above, birds that produce duets tend to be sedentary. The researchers also reconstructed the evolutionary history of duets, revealing that this behavior evolved several times (concentrated in particular genera, such as Myioborus and Myiothlypis). The ancestor of these birds probably did not sing duets.


The evolutionary history of duets and migration in the New World warblers. Birds that sing duets (red dots on the left) tend to be sendentary (blue dots in the right). From: Mitchell et al. (2019) The Auk


A role for hybridization?

One big assumption in this macro-evolutionary study is that the traits under investigation (here, duets and migration) follow the species tree. But this does not have to be the case (see this commentary in Evolution). If traits have a genetic basis, they could be exchanged between species by hybridization. On the evolutionary tree, it might seem like an independent origin, while in reality the trait was transferred during a hybridization event.

I don’t know whether this scenario applies to the duet behavior in New World warblers. These birds are known to hybridize extensively (see here for an overview), but the genetic basis of duets is – to my knowledge – still a mystery. It would be interesting to pinpoint the genes underlying this behavior and reconstruct their evolutionary history. Will they follow the species tree?



A duetting species from Costa Rica, the Collared Whitestart (Myioborus torquatus) © Cephas | Wikimedia Commons



Hahn, M. W., & Nakhleh, L. (2016). Irrational exuberance for resolved species trees. Evolution, 70(1), 7-17.

Mitchell, L. R., Benedict, L., Cavar, J., Najar, N., & Logue, D. M. (2019). The evolution of vocal duets and migration in New World warblers (Parulidae). The Auk: Ornithological Advances, 136(2), ukz003.


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