Science1 publisher2 min readPublished
Würzburg model puts New World blackbirds in South America about 6 million years ago
Würzburg biologists date New World blackbirds' first move into South America to about 6 million years ago, roughly 3 million years earlier than past estimates. Their model has the family starting in North America and crossing south before the Isthmus of Panama had fully formed.
The Scientist · Science desk
What happened
- The best-fitting scenario runs south in steps: Mesoamerica and the Caribbean about 7.5 million years ago, Amazonia 6.1 million, the Andes soon after, southern South America 4.8 million, Patagonia 2.7 million.
- A 2015 study had already challenged Ernst Mayr's 1946 South American origin for the family, but it left open how and when the birds spread across the Americas.
- Many species arose inside regions the family had already colonized, and speciation rates were broadly similar in North and South America.
Compiled by The ScientistSomething wrong?How this is made
Why it matters
- constraint Because the crossing predates a finished isthmus, any account of the route has to run over open water or across islands and early land links, and the paper lists these as possibilities without choosing one.
- constraint The grassland link is a match in dates, so the result cannot be cited as evidence that habitat change drove blackbirds south until habitat is tested as a variable in a model.
- decision Explaining why some icterid lineages produced more species now means building traits such as diet, habitat specialization, social behavior and mate choice into the models, since new territory did not predict where species arose.
Ernst Mayr started this argument in 1946, when he proposed that the family began in South America and spread north in steps [3]. Whether the new work closes it depends on what "most likely" means. In the Frontiers of Biogeography paper by Axel Arango Garcia and Chaitanya Gokhale of the University of Würzburg [1], it means the best fit among a range of statistical models run on distribution data and evolutionary trees [5][6]. The sample covers nearly the whole family: 104 of 106 recognized species, about 98 percent [1]. The phys.org account does not report how far the winning scenario finished ahead of the others. I'd call the North American origin the favored answer of a second analysis, after a 2015 study first pointed north [4], and I would stop short of calling the question closed.
The new part is the timing. The birds reached Mesoamerica and the Caribbean around 7.5 million years ago and Patagonia around 2.7 million years ago [6]. That is a spread of about 4.8 million years from first step to last [2]. The South American leg began around 6 million years ago [7]. "Our findings trace the spread of New World blackbirds across the Americas as a long, gradual process. What is particularly interesting is that their ancestors appear to have reached South America before the present-day Isthmus of Panama had fully formed," Arango Garcia said [14].
Open, grass-dominated habitats were spreading in South America at around the same time [9]. "The spread of icterids into South America seems to coincide with the expansion of open, grass-dominated habitats there. It is therefore possible that the birds followed these suitable habitats southward," Arango Garcia said [15]. The thing this doesn't tell you is whether the grass mattered. Birds following grassland and birds arriving while grassland happened to spread would leave the same dates, and the study cannot show that the habitat change caused the expansion [10].
On why the family split into so many species, the paper concludes that factors other than geography, possibly including ecological specialization, social behavior and sexual selection, are likely to play an important role [13]. The project began in teaching. Arango Garcia has used the Icteridae as a classroom example for years [16].
What to watch
- The paper's model-comparison scores, which would show how far the southward scenario leads a South American origin like Mayr's.
- Whether adding the two species missing from the 104-species dataset shifts any of the dated steps.
- Dated reconstructions for other bird families that entered South America around 6 million years ago, which would show whether the blackbirds' early crossing is unusual.