Similar migration strategies explain the evolution of flight calls in wood warblers

Although phylogenetic relatedness plays a major role as well.

Several species of wood warbler (family Parulidae) migrate at night. During their nightly travels, they produce short, simple calls known as “flight calls”. In some species, these flight calls are so similar that they have been grouped into bio-acoustic categories, such as the “Zeep complex” and the “Upsweep complex.” Interestingly, members of these complexes tend to exhibit similar migration strategies. Could there be an evolutionary link between these traits? Given that wood warblers often migrate in mixed species flocks, shared flight calls might facilitate interspecific communication during migration. In a recent Evolution study, Zach Gayk and his colleagues tested this hypothesis by comparing the flight calls and migration strategies of 36 wood warbler species.

Migratory Variables

The researchers quantified the similarity of flight calls in the different species and correlated this with several features of migration, such as overlap in wintering range and total migration distance. The analyses indicated that phylogenetic relatedness explains a large part of the variation in flight calls (similar to this study on the evolution of bird song). In other words, closely related species produce similar flight calls. After correcting for phylogenetic relationships, some migratory variables were associated with similarity in flight calls. When considering all 36 species, flight calls were similar between species that breed at similar latitudes and that show a temporal overlap in migration timing. A more detailed analysis on the “Zeep complex” found significant effects of migration length and overlap in wintering range on flight call similarity. Taken together, these findings support the hypothesis that migratory similarity is a driving factor in the evolution of flight calls.

Range maps and flight call spectrograms for six species of migratory wood warblers. The top three species have similar long-distance migrations and acoustically similar flight calls, whereas the bottom three species have more varied migrations and dissimilar flight calls. From: Gayk et al. (2021) Evolution.

Benefits

Intuitively, these findings make sense. Efficient communication during migration can be a life-saver. Literally. Similar flight calls can help birds to find high-quality stop-over sites, avoid predators, and reduce the chance of getting lost during migration. The researchers suspect that selection for similar flight calls was quite strong during the last 50,000 years of glacial cycles when “shifting migratory routes may have driven the need for increased communication.” However, the link between flight calls and migratory strategies remains to be tested in other bird species.

Finally, I cannot help but think of a possible role for flight calls in hybridization. Wood warblers are known for their propensity to hybridize (see the Parulidae page for an overview). Could this be related to the similarity in flight calls or migration strategies? Disentangling these two factors will be tricky, but worthwhile to explore.

References

Gayk, Z. G., Simpson, R. K., & Mennill, D. J. (2021). The evolution of wood warbler flight calls: Species with similar migrations produce acoustically similar calls. Evolution75(3), 719-730.

Featured image: Blackburnian warbler (Setophaga fusca) © Mdf | Wikimedia Commons

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