Three For The Price Of One: Genomics Reveals Three Distinct Cockatoo Species

An international collaboration of scientists have made a groundbreaking discovery that could positively impact the conservation of three iconic yellow-crested white cockatoo species

“In the end, we will conserve only what we love; we will love only what we understand and we will understand only what we are taught.”

― Baba Dioum (1937 - current)

A recent deep dive into the genomics of several cockatoo taxa by an international team of scientists has revealed these parrots comprise three distinct species — not subspecies, as often claimed. Until now, whole-genome research had not been carried out on any of these cockatoo species and thus, their taxonomy is rather messy, leaving people to distinguish between them solely on the basis of the parrots’ subtle — and often overlapping — physical differences.

This confusion is particularly concerning because one of these three cryptic species is critically endangered: the yellow-crested cockatoo population of New Guinea has only 2,000 individuals remaining in the wild, thanks to illegal trapping for the pet trade.

The first of these two studies focused on identifying whether the sulphur-crested cockatoo, Cacatua galerita is distinct from the triton Cockatoo, Cacatua triton. The sulphur-crested cockatoo, often known in aviculture as the greater sulphur-crested cockatoo, is a large-bodied white cockatoo that is native to eastern Australia from Cape York to Tasmania where it lives in a variety of diverse habitats. It is less arboreal than its other cockatoo cousins, and is more inclined to live in open or semi-open habitats, including fields, golf courses and lawns.

In contrast, the triton cockatoo appears to be exclusively a forest bird. Long misidentified as a subspecies of the larger galerita cockatoo, the triton cockatoo ranges across most of New Guinea, whereas the galerita cockatoo now is known to be restricted to just Australia and to a very tiny portion of southern New Guinea.

Although originally classified as two distinct species, for over a century the triton cockatoo had been confused to be a subspecies of the galerita cockatoo, due to their strikingly similar appearances. For this reason, it was erroneously thought this one “species” occupied a huge range throughout both Australia and New Guinea.

An international team of scientists just published two genomic studies of the yellow-crested white cockatoos. One of these studies reports that these two taxa are indeed, separate species that occupy separate landmasses (Figure 1).

Except it doesn’t.

This study, led by molecular ecologist and biogeographer Arthur Sands, a research fellow at the National University of Singapore and at the University of Hong Kong, where he studies avian diversity in New Guinea and Australia, used modern genomic analysis to establish that the triton cockatoo is its own distinct species that occupies most of New Guinea, whereas the galerita cockatoo is now known to be restricted to Australia and to a very tiny part of southern New Guinea (ref).

The importance of this distinction has serious implications for conservation, particularly in New Guinea where both species co-occur (in a tiny area) and where programs led by the Indonesian government and NGOs are designed to reintroduce surrendered pet cockatoos into the wild on the western part of the island to counter the pressures of climate change, habitat destruction, and illegal trapping for the pet trade.

“Introducing the wrong species in the wrong place could jeopardize their long-term survival in the wild through hybridization or competition between the Triton Cockatoo and the Sulfur-crested [galerita] Cockatoo, potentially even disrupting ecosystems in the long term,” Dr Sands explained.

In addition to genomic research, this study reports that bioacoustic analyses of audio recordings downloaded from Xeno-Canto reveals these two parrot species sound different (Figure 2).

“Our genomic and bioacoustic results consistently support the existence of two species, C. galerita and C. triton, within what has long been considered a single species, C. galerita,” the study’s authors wrote (ref). “At the same time, within C. galerita sensu stricto (s.s.), our analyses identify two subspecies corresponding to ESUs.”

An ESU, or “evolutionarily significant unit”, is a population of organisms that is considered distinct for the purposes of conservation.

Now that the triton cockatoo is once again restored to its rightful place as its own distinct species, it’s important that reintroduction programs for these cockatoos incorporate genomic data into their programs to avoid hybridization with other species.

“Consequences of hybridization are most of time unpredictable,” said the senior author of both studies, animal ecologist Juha Merilä, chair and associate director of Ecology & Biodiversity, and a professor at Hong Kong University as well as at the University of Helsinki.

“While both positive and negative outcomes are possible, the prevailing view is that we should exercise precaution to avoid possible negative consequences of hybridization (e.g. reduced fertility or hybrid breakdown),” Professor Merilä told me in email.

Additionally, this study’s taxonomic reclassification will require taxonomic updates by CITES, an organization whose purpose is to protect parrots and many other endangered species around the world. These updates may also require new levels of protection in view of this split.

The second of the two studies was led by conservation biologist Astrid Andersson, a postdoctoral fellow at the University of Hong Kong. Dr Andersson is continuing her conservation work with the yellow-crested cockatoo, Cacatua sulphurea, that she began as a graduate student by applying genomic techniques, in anticipation that she can use these data to learn exactly where Hong Kong’s urban cockatoos came from and to assess their future conservation value.

Distinguishing between these two cockatoo species is somewhat less challenging. For example, the sulphurea cockatoo is a noticeably smaller cockatoo than the galerita cockatoo. Unlike galerita cockatoos, the small sulphurea cockatoo lives in wooded and cultivated areas of East Timor (also known as Timor-Leste) and on Indonesia’s islands of Sulawesi and the Lesser Sundas. The sulphurea cockatoo is critically endangered with fewer than 2,000 individuals alive in the wild due to illegal trapping for the pet trade. Between 1981 and 1992, more than 190,000 sulphurea cockatoos were illegally trapped and exported from Indonesia for the cage-bird trade, according to Birdlife (more here). Currently, its population is thought to still be declining.

In this study, Dr Andersson and collaborators examined DNA that they extracted from museum specimens held by natural history museums in the USA and Europe to elucidate the genetic diversity amongst the presumed subspecies.

“One of the benefits of museomics is the ability to examine genetic data from taxa that are extinct, rare or inaccessible,” Dr Andersson pointed out.

“In this case, it provides valuable information to inform conservation efforts, such as translocation, genetic rescue and breeding — steps that are crucial to avoid global extinction of C. sulphurea.”

Using museomics, Dr Andersson and collaborators identified three genetically distinct groups of cockatoos that live throughout Wallacea, a biogeographical region located between the Asian and Australian continental shelves.

Surprisingly, Dr Andersson and collaborators were not able pick out the citron-crested cockatoo, C. s. citrinocristata, a cockatoo found on the island of Sumba. The citron-crested cockatoo is classified as a distinct species by some scholars and by the IUCN, based on the fact that it differs substantially from the other sulpharea cockatoo subspecies — notably it has a distinct orange (rather than yellow) crest and its unique juvenile beak coloration. So this “species” may not be a species afterall.

“Our inference is based on analysis of their genetic divergence from other putative C. sulpharea subspecies suggesting relatively low levels of divergence comparable to that among other cockatoo subspecies,” the study’s senior author, Professor Merilä told me in email.

Dr Andersson and collaborators also examined the genomics of Abbott’s cockatoo, C. s. abbotti, which has been confused to be a subspecies of the sulphurea cockatoo. This mysterious cockatoo lives in isolation on the remote Indonesian island chain, the Masalembu Archipelago. This archipelago is so tiny and remote that it usually is not identified on most maps. The Masalembu Archipelago lies far to the west of Wallace’s line in the Java Sea, giving Abbott’s cockatoo the distinction of being the furthest westward representative of any known population of cockatoos.

Dr Andersson and collaborators noted that although it is possible that cockatoos self-colonised Masalembu through overwater dispersal, it is equally likely that they were brought there by people following historical trade routes, since trade in cockatoos from this part of the world has recorded as far back as the 13th century (ref).

“[W]e do believe it was introduced there by humans, possibly as an accidental release of birds in trade,” Professor Merilä told me in email. “We are currently working to understand their origins better by sequencing more abbotti samples.”

Nevertheless, precisely how, when and why these cockatoos reached this remote archipelago remains shrouded in mystery.

Once again, this second study reminds conservation biologists of the challenges of distinguishing between cryptic species, especially those species that have been hybridized as a result of the pet trade, and the importance of properly identifying the taxa they are working with.

“Our research highlights the genetic diversity within and among these iconic cockatoo species and underscores the importance of incorporating genetic data into conservation planning,” Professor Merilä said.

A cockatoo species primer

(NOTE: only birders, aviculturists and other bird geeks will appreciate this information, which will not appear on the exam. In addition to the two studies I cover here, this data is also summarized from the Helm Field Guide to Parrots of the World by Joseph Forshaw (2010) and Parrots: A Guide to Parrots of the World by Tony Juniper and Mike Parr (1998). You will also note that both book references require significant updates.):

Sulphur-crested cockatoo, Cacatua galerita

The sulphur-crested cockatoo, Cacatua galerita, often known in aviculture as the greater sulphur-crested cockatoo, is a large-bodied white cockatoo that is native to eastern Australia from Cape York to Tasmania where it lives in a variety of diverse habitats. It more inclined to live in open or semi-open habitats, including fields and lawns. These cockatoos can be very numerous locally and are very intelligent — so much so that people may view them as troublemakers or pests (more here). The galerita cockatoos have a white periophthalmic ring, which distinguishes them from their smaller cousins.

In flight, the galerita cockatoo generally gives the impression of a large parrot with a large, slender body and long wings.

Triton cockatoo, Cacatua triton

The triton cockatoo has been long been thought to be a subspecies of the larger galerita cockatoo. Nevertheless, genomics work shows that the triton cockatoo is a distinct species. It is sometimes known as the medium-sulphur crested cockatoo in aviculture due to its intermediate size between the three now-recognized species. In the wild, the triton cockatoo’s range is limited to most of New Guinea, whereas the larger galerita cockatoo now is known to be restricted to Australia and to a very small portion of southern New Guinea. Unlike the galerita cockatoos, triton cockatoos appear to be exclusively a forest species, and they usually have bluish bare skin around their eyes

Its head and bill appear proportionally larger to its body when compared with the galerita cockatoo.

Yellow-crested cockatoo, Cacatua sulphurea

The yellow-crested cockatoo, Cacatua sulphurea, is the smallest of these three cryptic species, and is typically referred to as the lesser sulphur-crested cockatoo by aviculturists. For example, the sulphurea cockatoo has much smaller wings and a shorter tail than the galerita cockatoo.

Besides its smaller body size, the sulphurea cockatoo can sometimes be distinguished from the larger galerita cockatoo by the pale yellow patches that are usually (but not always) visible on its cheeks, making cheek coloring a somewhat unreliable marking, and also by the bluish-white bare skin around its eyes.

The citron-crested cockatoo, C. s. citrinocristata — either a species or a subspecies depending upon who you argue with — occurs on the island of Sumba, and has a distinctive orange crest and cheeks.

In flight, the sulphurea cockatoo’s overall impression is a middle-sized parrot with a proportionately large bill, somewhat large head, and small body.

The sulphurea cockatoo, which is found in wooded and cultivated areas of East Timor and on Indonesia’s islands of Sulawesi and the Lesser Sundas, is critically endangered due to illegal trapping for the pet trade. Currently, there are fewer than 2,000 individuals in the wild, and its population is thought to be declining. However, there may be some small hope for this species: it was accidentally introduced, probably via escaped pets, to Hong Kong where a population now thrives. The feral Hong Kong population is estimated to number around 200 individuals (more here) — 10% of the original Sumba population — and thus, may serve as a genetic rescue for the wild population provided, of course, that the Hong Kong population are not hybrids.

Sources:

Arthur F Sands, Astrid A L Andersson, Kerry Reid, Taylor Hains, Leo Joseph, Alex Drew, Ian J Mason, Frank E Rheindt, Caroline Dingle, and Juha Merilä (2024). Genomic and Acoustic Biogeography of the Iconic Sulphur-crested Cockatoo Clarifies Species Limits and Patterns of Intraspecific Diversity, Molecular Biology and Evolution 41(11):msae222 | doi:10.1093/molbev/msae222

Astrid A. Andersson, Arthur F. Sands, Kerry Reid, Taylor Hains, Paolo Momigliano, Jessica G. H. Lee, Geraldine Lee, Frank E. Rheindt, Juha Merilä, and Caroline Dingle (2024). Museomics Sheds Light on Evolutionary Diversity in a Critically Endangered Cockatoo Species From Wallacea, Molecular Ecology | doi:10.1111/mec.17616

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