“One of the most exciting discoveries I have been involved with” – Professor Per Alström
This was how Per described the results of his latest research, released last week.
Coming from the man who, among other things, discovered the breeding grounds of the enigmatic Blackthroat in China, not to mention several species of bird new to science, this was quite a statement.
And so, unusually for me, I read a scientific report from front to back. And it reminded me – as if I needed reminding – just how brilliant are nature and evolution.
Per’s report – based on DNA analysis – reveals that two little-known forest-dwelling birds are actually members of the pipits and wagtails family, evolving very different appearance and behavior after colonizing tropical-forested islands.
The Madanga Madanga ruficollis, occurring exclusively on the island of Buru, Indonesia, is actually a pipit (Anthus) and the São Tomé Shorttail Amaurocichla bocagi is a wagtail (Motacilla). The strikingly different appearance of these birds, compared with their closest relatives, has totally obscured their true relationships.
The Madanga, formerly considered an aberrant-looking white-eye (!) is actually a pipit, displaying plumage not resembling any of the world’s 40 species of Anthus. Photo by Rob Hutchinson/Birdtour Asia
A more ‘traditional’ pipit – a Tree Pipit (Anthus trivialis). Photo by Göran Ekström.
The Sao Tome Shorttail (Amaurocichla bocagi) is actually a wagtail. Photo by Fabio Olmos (www.arkive.org).
A more ‘traditional’ wagtail – a White Wagtail (ssp baicalensis).
As well as plumage, the Madanga and São Tomé Shorttail have different habitat choice and feeding style from pipits and wagtails. Both inhabit primary forest, where the former feeds like a nuthatch on epiphyte-covered branches and tree-trunks, while the latter feeds both on the ground and on tree trunks and branches. In contrast, nearly all pipits and wagtails occur in open habitats, and all forage exclusively on the ground.
The suggestion is that the radically different appearances of these birds were triggered by the fundamental shifts in habitat and feeding behaviour following their colonisation of forest-covered tropical islands. This is estimated to have happened around 4 and 3.3 million years ago, respectively.
The presumption is that the birds’ ancestors were long distance migratory species that landed on the islands and became established, despite the alien habitat.
“These islands were probably totally covered by forest when these birds’ ancestors first arrived, which is a very unfamiliar habitat to pipits and wagtails,” Professor Alström said.
Two of the closest relatives to the Madanga are the European Tree Pipit (Anthus trivialis) and the Olive-backed Pipit (Anthus hodgsoni), which, although breeding in forested areas, nest and feed on the ground.
“Although the Madanga’s plumage is very different, interestingly, the structure of the bird is quite similar to the Tree Pipit and we believe that the ancestor of the Madanga was pre-adapted to this new niche it became established within.” – Professor Per Alström
“Tree Pipits are very good at creeping through dense vegetation on the ground. When they are startled and flushed from the vegetation, they often fly up into a tree and they will sit there or they are able to walk quite effortlessly along bigger branches. But they never feed up in the trees, so the hypothesis here is that the ancestor of the Madanga would come to the island of Buru, which was completely covered in forest, and it might have fed on the ground between the trees but then would fly up into the canopy when it was scared by something. It would have then discovered that there was plenty of food on these epiphyte-covered branches and, as it was pre-adapted to walking along branches and in thick ground cover, it could probably have managed fairly well in that new habitat.”
Prof Alström said that this would have meant that there was no evolutionary pressure for it to evolve a new structure.
“For example, if you look at the bill, it is very similar to a Tree Pipit’s, and so are the legs and claws. This means the actual shape of the bird has not changed very much.”
For the full report, see: Alström P, Jønsson KA, Fjeldså J, Ödeen A, Ericson PGP, Irestedt M. 2015 Dramatic niche shifts and morphological change in two insular bird species.R. Soc. open sci.2: 140364. See URL: http://dx.doi.org/10.1098/rsos.140364
And Per Alström’s research page can be accessed here.