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By Andrea Wild Duncan Rae 26 September 2019 5 min read

THE Yellow Oriole is one of many birds that occur in the Trans-Fly, a region in the south of New Guinea known for vast coastal wetlands, savannas, grasslands and forests.

It’s one of the most ecologically diverse regions on the planet.

Paperbark forest in the Trans-Fly.

An unusual oriole

Dr Leo Joseph, Director of the Australian National Wildlife Collection (ANWC), was cataloguing bird specimens from a field trip to the Trans-Fly, when one stood out. Thought to be a Yellow Oriole, Oriolus flavocinctus, a species spread widely across the rainforests of northern Australia and extending into southern New Guinea, to Leo the bird in his hand looked like a hybrid.

Until that point, no one had suspected that Yellow Orioles might hybridise with other species. When Leo looked for potential suspects there weren’t many options. The two front runners were the Brown Oriole, Oriolus szalayi, and the Olive-backed Oriole, Oriolus sagittatus, but both species appeared to have an alibi.

“Brown Orioles aren't known to occur in the same areas as Yellow Orioles. Olive-backed Orioles do occur in the same areas as Yellow Orioles, but in northern Australia Olive-backed Orioles live in eucalypt savanna, which is very distinct from the rainforest habitat of Yellow Orioles,” Leo says.

“We wondered whether in New Guinea, the Trans-Fly were somehow causing these two species to come together and interbreed,” he says.

Hunting in collections

The first step was to search the world’s natural history collections for other potential hybrids among birds identified as Yellow Orioles or Olive-Backed Orioles.

Three museums hold extensive collections of orioles from the Trans Fly. They are the Papua New Guinea National Museum and Art Gallery in Port Moresby, the Naturalis Biodiversity Center in Leiden and the American Museum of Natural History in New York.

Only two of the specimens held in these museums looked unusual: a Yellow Oriole in New York that was collected from the Trans-Fly in 1937 and appeared similar to Leo's unusual specimen; and a Yellow Oriole in Port Moresby that was collected from the Trans-Fly in 1969 and had an unusual grey-blue colour.

If hybridisations were occurring in the Trans-Fly, they seemed to be very rare but interesting events.

The three aberrant orioles from the Trans-Fly held by the Australian National Wildlife Collection (left), American Museum of Natural History (middle) and Papua New Guinea National Museum and Art Gallery (right). Image first published in Emu - Austral Ornithology.

On the DNA Trail

Originating in Europe hundreds of years ago, taxonomy began as a visual science, with species described and differentiated by visible traits such as the colour and patterns of their feathers. Today scientists can sequence DNA from old museum species, seeking answers to thorny taxonomic questions at the molecular level.

“We found that our aberrant oriole specimen had mitochondrial DNA identical to that of Olive-backed Orioles,” Leo explains.

“Mitochondrial DNA is passed down only from mothers to their offspring, thus the specimen must have been descended from a female Olive-backed Oriole down the female line,” he says.

“As its plumage is clearly a combination of Yellow and Olive-backed Orioles, we think a female Olive-backed Oriole bred with a male Yellow Oriole and the female descendants of this hybrid continued breeding with Yellow Orioles to produce our aberrant Oriole,” he says.

More secrets spilled by DNA

The researchers went on to examine the mitochondrial DNA of six normal looking Olive-backed Orioles, including some ANWC specimens from the Trans-Fly. The results for five were unremarkable.

The sixth had plumage typical of Olive-backed Orioles, but its mitochondrial DNA revealed a different story. It was an identical match with Yellow Orioles.

“After ruling out mistakes or contaminated results, there was only one conclusion,” Leo says.

“A female Yellow Oriole has, at some point in the past, bred with a male Olive-backed Oriole. Since mitochondrial DNA is carried through the female line only, her daughters and their daughters must have continued breeding, or backcrossing as it is called, with Olive-backed Orioles. The birds receiving the Yellow Oriole mitochondrial DNA eventually came to be more or less indistinguishable visually from Olive-backed Orioles but they still carried Yellow Oriole mitochondrial DNA,” he says.

What’s unique about the Trans-Fly?

In Australia the dividing line between rainforest and eucalypt savanna is clear. In the Trans-Fly the borders are much less distinct. The habitat is intermediate between rainforest and savanna. It is known as swamp savanna.

“The place where these odd hybrid orioles lived in New Guinea could legitimately be called tall open Melaleuca forest,” Leo says.

“This unique habitat shares features of both the rainforest habitat of Yellow Orioles and the eucalypt savanna habitat of Olive-backed Orioles in Australia,” he says.

“There are some other possibilities, like low population numbers leading the birds to hybridise due to a lack of mates, but we think the unique habitat of the Trans-Fly is bringing the two species together and causing occasional hybridisation events.”

The aberrant oriole (sixth from left) surrounded by Olive-backed Orioles (at left), Yellow Orlioles (at right) and a Brown Oriole (far right) from Cape York Peninsula (CYP) and the Trans-Fly (TF).  The Australian National Wildlife Collection in Canberra is home to 60,000 bird specimens representing more than a thousand species. Image first published in Emu - Austral Ornithology.

We still don’t know how often oriole species interbreed or the story behind the unusual grey-blue specimen collected in the Trans-Fly in 1969 that is held in the Papua New Guinea National Museum and Art Gallery. But through good detective work in natural history collections and the use of modern DNA techniques, it is becoming easier than ever to investigate these questions.

Like many discoveries, this story began with a single scientist in a collection raising an eyebrow at something unusual. There are more than 15 million specimens in CSIRO's National Research Collections Australia. Many are waiting to reveal their secrets to researchers with a keen eye and questioning mind.

Aberrantly plumaged orioles from the Trans-Fly savannas of New Guinea and their ecological and evolutionary significance and the oriole photos included here were published in the scientific journal Emu - Austral Ornithology.

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