No new thing under the sun? Finding sunbird species in Wallacea

A male Wakatobi Sunbird

Our understanding of how species interact and evolve depends on accurate knowledge of the species that exist on Earth. There are still many species to be identified, however, even in evolutionarily significant regions such as Wallacea in central Indonesia, site of Alfred Russel Wallace’s pioneering work. Our new paper, completed jointly with researchers from Universitas Halu Oleo and just published in the Zoological Journal of the Linnean Society, draws on work carried out in Wallacea to identify multiple unrecognised species in the beautiful sunbird family. Made using modern genetic, acoustic, and statistical techniques, these discoveries add to our understanding of how life evolved in this region and reinforce some of Wallace’s original ideas.

The key discovery is the “Wakatobi Sunbird Cinnyris infrenatus”, a species endemic to the small islands of the Wakatobi archipelago, off Southeast Sulawesi in central Indonesia. The Wakatobi Islands have been separated from other landmasses since they first rose out of the sea, and so there has been plenty of time for their populations to evolve in isolation and produce endemic taxa, found nowhere else on Earth. The Wakatobi Islands have been recognised as a Key Biodiversity Area for their importance to the survival of biodiversity. The Wakatobi Sunbird is the latest endemic species to be identified by our research group, following previous work on the Wakatobi Flowerpecker and the double discovery of the Wakatobi White-eye and Wangi-wangi White-eye. It’s important that we know all of the species of Southeast Sulawesi and the Wakatobi Islands, because this region acts as a “natural laboratory” for the study of evolutionary processes such as cryptic sexual dimorphism, the “supertramp strategy”, and the links between behaviour and population divergence. The Wakatobi Sunbird is currently treated as a subspecies of the widespread Olive-backed Sunbird (Cinnyris jugularis), but our findings indicate that the Olive-backed Sunbird is actually made up of at least 4 reproductively isolated species.

The sunbirds (Nectariniidae) fill a similar niche in Africa, Asia, and Australia to the hummingbirds of the Americas. They are small birds with long bills that help them extract nectar from flowers. Like the hummingbirds, many sunbirds (males particularly) exhibit brightly coloured plumage, with beautiful iridescent or “metallic” feathers that reflect the sunlight. In fact, the naturalist William Jardine tells us in his 1843 volume on the sunbirds that sunbirds get their name “from their brightly-tinted dress, appearing in higher splendour when played on by the sun-beams”. For hundreds of years, ornithologists have used the patterns and colours of these feathers to identify sunbird species. Now, however, we can combine multiple forms of data to uncover patterns that weren’t clear from plumage alone. Our paper also looked at the Black Sunbird (Leptocoma aspasia), a species with male plumage that’s hard to examine because it mostly looks jet-black, except when the sun hits it in the right way to reveal other colours. We found a genetic split in this species that had not been suggested by any previous work, probably due to its plumage being less informative.

Finding species like these isn’t just interesting for its own sake. It is also our best evidence to understand how evolution produces new species. This is particularly interesting in a region like Wallacea, which played such a significant role in the development of evolutionary biology. The observations that Wallace made around this region led him to discover evolution by natural selection, work which was published jointly with Charles Darwin in 1858 in the same journal where our sunbird paper has just appeared.

One of the observations that inspired Wallace’s evolutionary thinking was the importance of biogeographic barriers. Wallace noticed that the animals found on Sulawesi are markedly different from those on neighbouring Borneo, evidence that species would evolve on one island and then have difficulty crossing over. This boundary came to be known as Wallace’s Line. We now understand that it represents the beginning of the deep waters of the Wallacea region, which persisted even when sea levels were lower, unlike the shallower waters of the adjoining Sunda Shelf which gave rise to land bridges. A similar barrier to the east came to be known as Lydekker’s Line. As seen in the map below, the range of the Olive-backed Sunbird (in yellow) is currently thought to cross both Wallace’s Line and Lydekker’s Line, while the Black Sunbird (in purple) stops at Wallace’s Line but crosses Lydekker’s Line. It’s quite remarkable to imagine these dainty little birds maintaining gene flow across barriers which block so many other organisms! Our work, however, has indicated that the Olive-backed Sunbird populations on either side of Wallace’s Line actually represent separate species. The same is true of Black Sunbird populations divided by Lydekker’s Line. Modern evidence has actually reinforced Wallace’s original ideas, showing once again that these Lines represent significant biogeographic barriers that block gene flow in most animals.

Map showing populations sampled for our new paper. We found evidence that the Olive-backed Sunbird is actually composed of separate species in the Philippines (“Garden Sunbird”). the Sunda Shelf (“Ornate Sunbird”), the Wakatobi Islands (“Wakatobi Sunbird”), and the islands from Sulawesi to the Sahul Shelf (“Sahul Sunbird”). We also found that Black Sunbirds in New Guinea are strongly genetically divergent from those in Sulawesi. These findings reinforce the importance of biogeographic barriers like Wallace’s Line and Lydekker’s Line to evolution.

Both evolutionary biologists and ecologists are gaining new insights from large datasets on the traits and genomics of species. However, as these datasets are organised by species, they rely on our species lists being accurate in the first place. Data from  “species” like the Olive-backed Sunbird or Black Sunbird might prove misleading, as each of these actually represent multiple species. Meanwhile, a small population like that of the Wakatobi Sunbird may not be included in such a dataset at all if it isn’t recognised as a species.

To quote an 1863 paper by Wallace, the world’s species represent “the individual letters which go to make up one of the volumes of our earth’s history and, as a few lost letters may make a sentence unintelligible, so the extinction of the numerous forms of life which the progress of cultivation invariably entails will necessarily render obscure this invaluable record of the past”. It is therefore an irrevocable loss to the world, to humanity, and to science when a species goes extinct while still unrecognised, “uncared for and unknown”.

Hoga, one of the Wakatobi Islands where sunbirds were sampled for this study. It has been a privilege to see such beautiful places and animals for this research.

While the Wakatobi Islands are biogeographically “remote” due to their small size and the permanent water barriers that surround them, it is worth noting that they are not the stereotypical “desert islands” Western readers may imagine. The islands have been part of important shipping lanes since at least the 14th century, and the people of the Wakatobi are known for their maritime traditions and unique language. As Dr David Kelly, the second author on the recent paper remarked: “The identification of the Wakatobi Sunbird serves to remind us that biodiversity is everywhere. This bird wasn’t found in a remote rainforest, but along the scrubby margins of busy towns and villages. Let us hope the children of the Wakatobi will be able to enjoy these special birds for generations to come.”

A relief from Borobudur Temple on Java, built over 1000 years ago. The birds in the yellow boxes were identified as Olive-backed Sunbirds by Ashari et al. (2021). Their fascinating paper is available at https://li01.tci-thaijo.org/index.php/tnh/article/view/253401

The beauty of the sunbirds has attracted scientists and artists for many years. Elsewhere in Indonesia, Java’s Borobudur (the largest Buddhist temple in the world, constructed in the 8th or 9th century CE) displays carvings of Olive-backed Sunbirds drinking nectar on its walls. The researchers who identified these carvings hypothesise that this symbolises the Buddhist ideal of enlightenment. Over a thousand years later, sunbirds are still enlightening us on the origin of species.

Female (top) and male (bottom) Olive-backed Sunbirds, from Shelley’s monograph, published 1876-1880 and available from the Biodiversity Heritage Library at https://doi.org/10.5962/bhl.title.53516. Our new paper has found that the Olive-backed Sunbird is actually made up of at least four separate species.

To find out more, read our paper in the Zoological Journal of the Linnean Society here: https://doi.org/10.1093/zoolinnean/zlac081

Tramps in Transition: Wallacea’s monarch flycatchers and their evolutionary natural experiment

Pale-blue Monarch on the left and Island Monarch on the right

A warm welcome back to all our readers! The new year is now well and truly upon us and we hope you’ve all had a safe and energised return to work. This blog is written by Fionn Ó Marcaigh, summarising his new paper. Congratulations Fionn and we hope our readers enjoy learning about your research as much as we have! So without further ado…

Science is about making observations from the natural world, drawing up hypotheses to explain the patterns you’ve observed, and then testing these hypotheses by experimentation. We tend to imagine scientists in white coats doing experiments in the lab, but our understanding of evolution also owes a lot to work done in “natural laboratories” like islands and other isolated habitats, where evolution has taken place under different conditions. Our new paper, just published Open Access by the International Biogeography Society in their journal Frontiers of Biogeography, has used an important natural laboratory in Southeast Asia to test a classic hypothesis based on a bird called the Island Monarch (Monarcha cinerascens). We’ve made observations that contradict parts of the hypothesis and discovered a possible new species in the process!

Our natural laboratory was a collection of islands around a region known as Wallacea in central Indonesia (see map below). Named after Alfred Russel Wallace, this is where he co-discovered evolution by natural selection while travelling around islands of all shapes and sizes, with the waters around them being so wide and deep that most species have trouble crossing them. Some organisms are better at crossing these barriers than others, with the Island Monarch thought to be particularly adept. As its name suggests, the Island Monarch is one of the kings of small islands. It can be found all the way from the islands off Sulawesi in Wallacea, to the farthest reaches of the Melanesian islands east of Papua New Guinea, but is missing from large islands like Sulawesi and New Guinea themselves.

Continue reading “Tramps in Transition: Wallacea’s monarch flycatchers and their evolutionary natural experiment”

The Bird Life of Wawonii and Muna Islands Part II: mining the literature, mining the hills

Indonesia is a constellation of islands, big and small and many-shaped, born out of volcanism or coral build-up or sheared from the ground of continents, each its own world. The complex geography of this region begat an equally complex biogeography, with evolution working enthusiastically to populate it, and the staggering biodiversity that resulted will surely occupy biologists for centuries to come (as long as it lasts that long). Every nook and cranny is filled with fascinating plants and animals, and our research group recently published a paper on the avian contents of a couple of those nooks and crannies. This post follows one by Darren on the fieldwork stories behind that paper, and the expeditions to remote Muna and Wawonii islands.

Muna and Wawonii in their corner of Indonesia. The numbers mark sites surveyed for the recent paper.
Continue reading “The Bird Life of Wawonii and Muna Islands Part II: mining the literature, mining the hills”

The Bird Life of Wawonii and Muna Islands Part I: biodiversity recording in understudied corners of the Wallacea region

With Indonesia I’ve always felt like I’m just scraping the surface. Even after five field seasons in that incredible country I feel I’m always just finding further questions. One of my favourite bits was just driving around to new sites and travelling to different islands. There are always feats of incredible daring to observe (just piloting a car on a rural road takes nerves of steel), and engineering marvels and follies cut out of rainforest or carved from the hills. Any local leader could seemingly achieve anything with sufficient ambition, the consequences of which could really go either way! Easy explanations are usually not forthcoming. It is a culture where much is left unsaid, where one must ask the correct question the correct way to receive an answer. Perhaps it is this sense of mystery that keeps me coming back year after year, with each new island a world in itself.

Continue reading “The Bird Life of Wawonii and Muna Islands Part I: biodiversity recording in understudied corners of the Wallacea region”

PhD Retrospective: Finding new bird species in Sulawesi

Almost as long as I can remember, I’ve wanted to be a Zoologist. Growing up on a steady diet of Attenborough documentaries, I dreamed of exploring new lands, discovering weird and wonderful species. I wanted to be the next Darwin. The next Wallace. In a lot of ways, my PhD research has been the fulfilment of that dream. It all started with my undergraduate thesis project in the Trinity College Dublin (TCD) Zoology Department. When Prof Nicola Marples and Dr David Kelly offered a project studying speciation on islands, I jumped at it. I spent the summer before the final year of my undergraduate island hopping in south-east Sulawesi, Indonesia, collecting data on how the physical traits of sunbird species had changed on isolated islands. Nicola and Dave’s project fieldwork was organised by Operation Wallacea, so we worked with Operation Wallacea staff as we moved around the islands and got to interact with scientists focusing on some of the other unique wildlife of Sulawesi.

Sulawesi is a region of massive interest to evolutionary biology. It is the intersection where Asian and Australian flora and fauna meet, and is separated by deep ocean trenches ensuring it has never been connected to a continent, allowing many unique species to develop there. As well as providing a fascinating study system, South-east Sulawesi has some incredible wildlife. Knobbed Hornbills, Tarsiers and Sulawesi Bear Cuscus are weird and wonderful treats. In particular, working with the birds of this region was incredible, and I was adamant it wouldn’t end up being a once in a lifetime experience! As well as allowing me to follow my island-hopping ambitions, this project kicked off what I predict will be a lifelong obsession. Having previously been a typical fan of large charismatic mammals, I became a full convert to the world of birds (it’s fundamentally all about the birds!).

Read the full post on the Operation Wallacea blog!