Birds of the lost arcs

By Dr Paul Oliver, Curator of Vertebrates at Queensland Museum

Occasionally at night as my family are sitting down to eat our dinner or doing the dishes, we hear a low, repetitive, booming, noise from somewhere outside. This noise is very easy to overhear, and dismiss as part of the background hubbub of suburbia. And the bird which makes this noise is also easy to overlook. Until you see it. And then it is unmistakable.

It is the frogmouth. Frogmouths spend their days perched motionless and doing a damn fine impersonation of a tree stump. But when startled they reveal themselves to be large birds with comical eyes and gigantic gaping beaks.  At night they come to life hunting insects and other small animals.

Tawny Frogmouth, Podargus strigoides. Image by Bruce Cowell.

In suburbia in Australia we have the Tawny Frogmouth. But this is just one of 13 odd species in that occur across Australia, New Guinea and tropical Asia. In many ways the Tawny Frogmouth is the outlier in the group. It has a wide distribution across open woodlands and savannahs. But the vast majority of other frogmouths are restricted to tropical rainforests, and some such as Solomon’s Frogmouth are very rare.

Where from frogmouth?

Amongst bird scientists, Australia is famous as a source area for the early evolution of some the world’s most distinctive birds. As you sit in your garden (as you may have been doing at lot more than normal of late) and watch honeyeaters, lorikeets or crested pigeons – you are looking at recent iterations from lineages that have very long histories in our region, going back tens of millions of years. Indeed, in the case of honeyeaters, they are early offshoots on a lineage of songbirds that moved out of Australia over 20 million years ago, through Asia, and went on to become the most diverse group of birds in the entire world.

So where does the frogmouth sit in this story? When and how did they move between Australia and Asia? In a paper recently published in the journal Biology Letters we try to address that question, and the answer is not simple.

Tawny Frogmouth, Podargus strigoides. Image by Jeff Wright.

In this work we generated genetic data for all three living genera (think of these as major evolutionary groupings) of frogmouths – Batrachostomus from Asia, Podargus from Australia and New Guinea, and Rigidipenna from the Solomon Islands. We then used this molecular data to estimate their evolutionary relationships and timeframes of separation.

Our analyses of genetic divergences across the three living genera of Frogmouths suggest they diverged around 30 million years ago. While molecular dating is an imprecise art, the splits between frogmouth genera are amongst the oldest estimated between bird lineages occurring in Australia and Asia. To put this in broader context, 30 million years ago Australia was separated from Asia by a wide ocean and had a very different (more temperate) climate. Making the jump between regions would be been challenging, especially for birds like frogmouths that tend to be fairly ‘stay-at-home’, and are probably disinclined to fly over hundreds of kilometres of open ocean. So how did they do it?

Tawny Frogmouth, Podargus strigoides. Image by Bruce Cowell.

The two-part tale of the frogmouth?

Our theory to explain how frogmouths made this jump has two parts. The first involves answering the question: which way did frogmouths jump? From Asia to Australia, or Australia to Asia?

There is a lot about the evolutionary history of frogmouths we don’t know, but they have been nice enough to leave us a few clues about where their early evolution occurred. Surprisingly, it’s neither Australia nor Asia where frogmouths live today, but North America and Europe! And while the Australian bird fossil record has major gaps, what information scientists have collected provides no evidence that frogmouths have a long history here. This hints at the answer to the first bit of the frogmouth story – they probably came from north, moving from Europe to Asia, and then into Australia.

The second part of the question then becomes, how did they make this jump across the oceans between the (then widely separated) landmasses of Asia and Australia?

A hint to the answer of this question comes from the aforementioned rare and unique frogmouth of the Solomon Islands. Our analyses indicate the ancestors of the Solomon’s Frogmouth diverged from all other living frogmouth 30 million years ago. This remarkably deep divergence suggests that frogmouths have also been in the south-west Pacific for a long time. Many models of geology also suggest that island arcs have been forming and moving across the south-west Pacific for over 35 million years. Some of these models even suggest there may have a chain of islands over a 1000 kilometres long.

So our theory is that frogmouths may have used ancient island arc systems, to first move out Asia, and then as staging grounds for subsequent colonisation of Australia.

Tawny Frogmouth, Podargus strigoides. Image by Gary Cranitch.

Resolving the history of a remarkable biota

There is a lot more water to go under this bridge. Many scientists remain sceptical about the roles of island arcs in the Australasian bird story. Conversely, some of us have argued that the role of island arcs in shaping the evolution of the Australian animals and plants has been systematically overlooked.

Certainly, different groups of birds and other animals will tell different stories and their remain great uncertainties. But these discoveries and hypotheses set the stage for new chapters in the story of understanding how Australia’s remarkable biota was assembled.

And on a more day-to-day level, next time a frogmouth lands on your clothesline, take a step back and think of the remarkable 30 million year journey its ancestors may have taken to get there.