Birds on a raft?

In the last post I brought up the case of New World suboscine birds, which, somewhat unexpectedly, have remained stuck in the Americas for some 50 million years. To balance out that example, consider the deep history of Hoatzins, weird birds that eat mostly leaves and, as nestlings, have claws on their wings that they use to clamber around in vegetation.

Hoatzin in Peru, looking vaguely prehistoric. Photo by Kate via Wikimedia Commons.

Hoatzins (Opisthocomus hoazin) live in the Amazon and Orinoco drainages of South America and are the only living members of the family Opisthocomidae. Until recently, fossil opisthocomids were also only known from that continent. But in 2011, a paleontologist named Gerald Mayr and two colleagues published a re-examination of some 17-million-year-old fossils from Namibia, and convincingly argued that they belonged to an opisthocomid. So that means the group occurred in both South America and Africa. And that leads to an obvious question: how did they get from one of those continents to the other?

The group is probably too young to have been affected by the opening of the Atlantic Ocean, so the obvious answer is that these birds colonized the New World by flying across that ocean, just like a dozen or so other bird groups are thought to have done. But there’s a big problem with that explanation: Hoatzins are very poor fliers, and Mayr and colleagues argued that this was also true of the known fossil opisthocomids. These birds couldn’t possibly have flapped or soared across an ocean.

Hoatzin in Bolivia, laboriously flying. A bird like this could not have flown across the Atlantic. Photo by Warren H via Wikimedia Commons.

Instead, Mayr et al. suggested a monkey-like solution to this monkey-like conundrum: the ancient ancestors of Hoatzins found themselves on a natural raft and drifted from Africa across the Atlantic. Hoatzins often gather in large groups along rivercourses, so the notion that their possible African ancestors might have gotten stranded on a giant piece of riverbank that broke off and floated to the ocean maybe isn’t so crazy. Plus, if those ancestors were leaf-eaters like the Hoatzin, a few trees growing on the raft could have provided them with enough food for the journey.

This case isn’t as convincing as the Atlantic-crossing monkeys, mainly because the fossil record of opisthocomids is very spotty; it could be that the group was more widespread than the known record suggests, and that might indicate other explanations for why they occurred on both sides of the Atlantic. Nonetheless, it’s an example to keep in mind when thinking about how birds have colonized new and far-off areas. They didn’t always have to fly.

Reference

Mayr, Gerald, Herculano Alvarenga, and Cécile Mourer-Chauviré. 2011. Out of Africa: Fossils shed light on the origin of the hoatzin, an iconic Neotropic bird. Naturwissenschaften vol. 98, pp. 961-966.