Explaining the pantropical distribution of kingfishers, bee-eaters, rollers, motmots, and todies

Scientists reconstruct the evolutionary history of the Coraciiformes.

Some bird groups have a peculiar distribution pattern. Take the order Coraciiformes, for example. This colorful bird order (which includes kingfishers, bee-eaters, rollers, motmots, and todies) has representatives in all equatorial regions, from South America over Africa to East Asia. Biogeographers refer to this distribution as pantropical and have formulated two main hypotheses to explain these patterns.

The first hypothesis states that the continental break-up of the southern supercontinent Gondwana divided widespread populations over the current tropical regions. The second hypothesis proposes that pantropical clades originated on the northern supercontinent Laurasia and later moved to southern regions, following the spread of tropical forests (that is why this hypothesis is also known as the Boreotropics hypothesis).


A Lilac-breasted Roller (Coracias caudatus) in Tanzania. © Jente Ottenburghs


Ultraconserved Elements

Jenna McCullough and her colleagues investigated which of these two hypotheses explains the distribution of the Coraciiformes. The obtained DNA from all extant coraciiform species and sequenced more than 5000 ultraconserved elements (UCE). These conserved DNA sequences are found in distantly related animal genomes and are probably involved in controlling gene expression. Their slow evolution can be used to probe evolutionary relationships in the distant past. Using several well-known fossils, the researchers managed to construct a time-calibrated evolutionary tree for the Coraciiformes. In addition, they reconstructed the ancestral distributions of these birds to pinpoint the location of their origin: Gondwana or Laurasia?


The evolutionary history of the order Coraciiformes. From: McCullough et al. (2019) Proceedings of the Royal Society B



The analyses revealed that the major coraciiform lineages originated on Laurasia about 57 million years ago. As the climate changed, the birds tracked the tropical conditions and forest habitats southwards, ending up in their current pantropical positions. These results support the Boreotropics hypothesis outlined above. During the Miocene and Pliocene, a second burst of diversification occurred, giving rise to the genera we know today.

Reading this study, reminded me of a blog post that I wrote for the BOUblog (the official blog of the ornithological journal Ibis) about the evolution of Trogons. Here is an excerpt from that post:

These analyses indicated that Eurasia is the most likely site of origin for these birds. The ancestors of present-day trogons were probably distributed across Laurasia (i.e. Eurasia and North America). During the transition from the Oligocene to the Miocene, about 23 million years ago, the climate cooled, forcing the trogons to move to tropical forests in the southern latitudes of the Neotropics, Africa and Asia. There, they thrived while their relatives disappeared from the Northern Hemisphere, culminating in the widespread distribution we observe today.

Another example of the Boreotropics hypothesis!



McCullough, J. M., Moyle, R. G., Smith, B. T., & Andersen, M. J. (2019). A Laurasian origin for a pantropical bird radiation is supported by genomic and fossil data (Aves: Coraciiformes). Proceedings of the Royal Society B, 286(1910), 20190122.

Oliveros, C. H., Andersen, M. J., Hosner, P. A., Mauck III, W. M., Sheldon, F. H., Cracraft, J., & Moyle, R. G. (2020). Rapid Laurasian diversification of a pantropical bird family during the Oligocene–Miocene transition. Ibis, 162(1), 137-152.

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