Are Homebody Warblers More Likely to Sing Together?

Guest post by researcher Liam Mitchell

Linked paper: The evolution of vocal duets and migration in New World warblers (Parulidae) by L.R. Mitchell, L. Benedict, J. Cavar, N. Najar, D.M. Logue, The Auk: Ornithological Advances.

Scientists who want to study the evolution of behavior face a fundamental problem: unlike bones, behavior generally doesn’t fossilize. However, that doesn’t mean that extinct species’ behavior doesn’t leave any evidence. The behavior of living or “extant” species can give us clues about the behavior of their ancestors, and we can use the behavior of living species, the evolutionary relationships among species, and computational modelling to make inferences about extinct species’ behavior.

In our study, we used this approach to study the evolution of vocal duetting and migration in New World warblers. Vocal duetting is when a mated pair of birds sings together. Duets are a cooperative behavior, because they communicate that the duetting pair will cooperatively defend their shared territory against intruders. It’s hard for mated pairs to stay together through migration, so non-migratory birds tend to have longer-lasting pair bonds than migratory species. These longer pair bonds mean that non-migratory birds may have more to gain from cooperative behaviors like duetting, so we might expect duetting to be evolutionarily associated with the absence of migration. 

We tested whether migrating and duetting are correlated in the evolutionary history of New World warblers. Essentially, we were looking to see if duetting and the absence of migration show up in similar places on the birds’ family tree. We collected data on each warbler species and determined whether or not they performed duets and whether or not they migrated. We used these data to perform our analyses.

Our primary analysis generated an evolutionary tree that shows duetting and migration over evolutionary time. We borrowed an existing phylogenetic tree of New World Warblers (Lovette et al., 2010) and used a technique called Markov Chain Monte Carlo (MCMC) to simulate trait evolution (Revell, 2013). MCMCs are computer simulations that calculate the frequency at which a given node (a point where the tree branches) exhibits a specific characteristic over a number of simulated generations. The likelihood that a given generation exhibited a characteristic is informed by the other nodes on the tree, especially the nearby ones. The frequency with which a node exhibits a characteristic in the simulation can be interpreted as the probability that the ancestral species at that node exhibited the characteristic. For example, if we calculate one million simulations for the most ancestral node on the tree, and 800,000 of those simulations exhibit the characteristic “migration,” we can say there is a high likelihood (0.8) that the last common ancestor of all living warblers migrated.

This let us calculate the evolutionary correlation between duetting and migration. Our analysis showed that migration and duetting are indeed negatively correlated over evolutionary time. In other words, duetting is associated with a non-migratory lifestyle, as we predicted. These methods allow us to get a quantitative look at the evolution and loss of certain behaviors over time. We can then draw informed conclusions about the nature of these complex behaviors and open the way for studying the factors that may have influenced their changes over time.

Read more on the lab website.

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