The world's biggest cuckoos once roamed the Nullarbor Plain

Western Australia’s Nullarbor Plain may be a vast treeless expanse today, but hundreds of thousands of years ago it was home to an array of weird and wonderful species, including two newly discovered extinct “giant cuckoos”.

The two species belonged to a group of birds called coucals, which are part of the cuckoo family. The larger of the extinct species would have stood more than half a metre tall and, judging by its bones, probably couldn’t fly.

The discovery, which we made during a 2014 dig at the Thylacoleo Caves, is not the first bizarre species found beneath the Nullarbor. Over the past decade, excavations of these caves have revealed thousands of exceptionally well-preserved fossils. Some are the remains of living species, others are of extinct species already known to science, while others have been totally new discoveries.

One surprise was finding two new species of tree-kangaroo. It takes a leap of imagination to envision today’s flat, treeless Nullarbor Plain covered in trees with large marsupials clambering about overhead. Clearly the region’s climate and conditions have undergone huge shifts – a fact underlined by the latest fossil finds.

Fossils found, species lost

Of the two new bird species we discovered, the smaller, Centropus bairdi, was similar in size to the largest living coucals from Melanesia, which weigh 700g or more. However, its exceptionally weak wing muscles imply that Centropus bairdi was flightless. We excavated its bones from cave sediments that are more than than 780,000 years old.

The larger species, Centropus maximus, is the largest cuckoo known anywhere in the world. It had long, powerful legs, and probably weighed well over 1kg, perhaps even topping 2kg.

Cavers first found one femur shallowly buried in a rock pile. Returning for more bones the following day, they uncovered two coucal skeletons side by side, where they presumably had lain undisturbed since the birds fell into the cave together hundreds of thousands of years before.

The two new species join another known extinct coucal, Centropus colossus, which was discovered in a South Australian cave 30 years ago, and was just a little smaller than Centropus maximus.

In their day, these two largest coucals would have been among Australia’s heaviest land-hunting birds. Today, the only living bird predators that can match them are the Wedge-tailed Eagle, Black-breasted Buzzard, White-bellied Sea-eagle and Powerful Owl.

All three coucals lived during the Pleistocene epoch, 2.5 million to 11,700 years ago. This was a time of ecological upheaval around the world. Successive ice ages caused sea levels to rise and fall, and temperatures and rainfall to fluctuate.

Many Pleistocene animals worldwide, particularly the large ones, went extinct as a result of climatic fluctuations, human impacts, or a combination of both.

We don’t know when these coucals went extinct, or what killed them, but this is one group where we can probably discount human hunting, as today’s coucals are reported to taste and smell appalling. It seems more likely that they died out when their habitat changed.

Birds of paradox

Living coucals are predatory, primarily ground-dwelling, and are known for their weak and graceless flight. They eat large invertebrates and small vertebrates, especially frogs. Perhaps the extinct ones tucked into the frogs that we now know lived on the Nullarbor too.

Coucals aren’t what we think of as typical cuckoos, which are best known for sneaking their eggs into the nests of other birds. Coucals (and many other members of the cuckoo family) build nests and raise their own young.

Even more unusually, female coucals are larger than males, and males do most or all of the work to raise the brood, a trait they share with only 5% of the world’s bird species.

The 26 living species of coucal span Africa, Madagascar, Asia, New Guinea, and northern Australia. For birds that have trouble staying airborne, they have managed to get around remarkably well.

Australia’s only living species, the Pheasant Coucal, is found only in the continent’s north and east. Without the fossil discoveries, we would never have guessed that their relatives once lived thousands of kilometres further south.

Rare as hen’s teeth

Few extinct birds are known from Australia, and only ten Pleistocene species have previously been described.

This is meagre compared to the 80 Pleistocene mammal species known to have been lost from Australia. This could mean one of two things: either birds cruised through the Pleistocene largely unscathed, or we have underestimated their rates of extinction.

The best-known extinct Pleistocene bird from Australia is the “thunderbird”, Genyornis newtoni, which stood 2m tall and weighed more than 200kg. The other nine Pleistocene species include flamingos from Lake Eyre, large megapodes related to the malleefowl from eastern and southern Australia, logrunners and a pardalote from Victoria, dwarf emus from King Island and Kangaroo Island, and the previously discovered giant coucal from South Australia.

Given such modest numbers, the two new Nullarbor coucals increase the number of known Pleistocene bird extinctions in Australia by 20%. Their disappearance is another piece in a complex ecological puzzle that covers just one corner of a vast continent.

Time will tell if these birds were local oddities, or the tip of an extinction iceberg that affected birds Australia-wide and has so far slipped under the radar.