Old World Warblers

For​ millions of years, male birds-of-paradise have been striking courtship poses in the forests of New Guinea. The black sicklebill perches on a stump, opens his shoulder plumes and tips forward, stretching his tail feathers out, like an obsidian comet. The blue bird-of-paradise hangs upside down from a branch and flounces his aquamarine gown. The twelve-wired bird-of-paradise bounces his banana-yellow backside, while Carola’s parotia sports a black-feather tutu and six-step dance routine.

Western naturalists have long regretted that such spectacular showmen are to be found only in New Guinea. ‘Nature seems to have taken every precaution that these, her choicest treasures, may not lose value by being too easily obtained,’ Alfred Russel Wallace wrote in 1862. New Guinea’s birdlife is not a secret the island has locked away in its forests, however, but a gift it has spread around the world. According to The Largest Avian Radiation: The Evolution of Perching Birds, or the Order Passeriformes, the island was central to the evolution of songbirds, the most conspicuous and widespread vertebrates on land.

There are more than 4500 species of songbird, and they live on every continent except Antarctica. What birds-of-paradise have in common with all other songbirds, such as thrushes, finches, wrens, crows and jays, is not the sound they make but the structure of their vocal organ – the syrinx – and their ability to learn new noises and expand their repertoires. Naturalists first recognised songbirds as an evolutionary unit in the 19th century, when men like Wallace collected animals from around the globe for European museums. Curators saw that many of the small birds from the Americas, Australia, Africa and Asia shared a feature with common European songbirds: a ‘perching foot’, with three toes pointing forward, a strong backwards-facing toe called a hallux and a separate tendon operating each. The foot became a key diagnostic character for the order Passeriformes, which quickly outnumbered all the other avian orders put together. By the end of the 20th century, the 6200 passerine species accounted for nearly two-thirds of total avian diversity. They are a prime example of what evolutionary biologists call a ‘radiation’ – the explosion of a single ancestral species into a multitude of forms.

For all their variety in plumage and behaviour, however, the different types of passerine bird exhibited few of the underlying anatomical differences on which traditional taxonomy relied. The complexity of the syrinx revealed an evolutionary division between the oscine passerines, or songbirds, and the 1300 species of suboscine passerines, which live mostly in tropical South America and lack vocal learning. But ornithologists struggled to sort Passeriformes into families. According to The Largest Avian Radiation, ‘the passerines were found to be so uniform in their anatomy that it was not considered possible to trace clear patterns of evolutionary descent among them.’

This changed in the late 1990s with the rise of molecular systematics, which uses DNA to determine the relatedness of species. The Largest Avian Radiation draws on hundreds of research papers to place almost every passerine species on an authoritative tree of life, synthesising ‘enormous amounts of data which previously were scattered on millions of book pages, or on labels of millions of specimens in museum collections, in huge databases in research institutions’. This has allowed the authors to map the dispersal of songbirds over millions of years, from their first emergence in the southern hemisphere to their worldwide distribution today. The book demonstrates on a global scale Wallace’s key observation after his travels in Indonesia and New Guinea more than 150 years ago: ‘The changes of the forms of animals,’ he wrote, ‘appear to have accompanied, and perhaps to have depended on, changes of physical geography, of climate or of vegetation.’

Passeriformes were probably the last avian order to evolve, splitting from parrots during the Paleogene period, around sixty million years ago, when Antarctica was temperate and still connected to Australia and South America. They spread across all three continents and probably thrived in dense understorey vegetation, bouncing between branches with their small bodies and strong feet. Cooling temperatures drove the Antarctic passerines to extinction around 35 million years ago, while the populations of New Zealand, South America and Australia were carried into isolation in the South Seas. The two surviving species of New Zealand wren, which superficially resemble the songbird wrens, have a relatively simple foot and syrinx and ‘represent a surviving lineage from the ancestral passerine stock, which lived in the ancient austral continent’. The South American passerines diversified into most suboscine species, including many of the beautiful birds, such as manakins and cotingas, of the Andes and Amazon.

The discovery of a southern origin for passerines, and songbirds in particular, was surprising. The evolutionary biologist Ernst Mayr argued in the mid-20th century that songbirds must have originated in the northern hemisphere and only later reached Australia from Asia. But DNA studies show that the oldest songbird lineage is the lyrebird of Australia, ‘nature’s tape recorder’. The next ten oldest songbird families are also Australian. The simplest explanation for so many Australian species at the base of the songbird tree is that the first songbirds were Australian too.

What happened next remains subject to debate. One argument is that passerines diversified in Australia, when the climate was still humid and tropical, and that New Guinea preserves this lost biodiversity. The authors of The Largest Avian Radiation have a different theory. ‘Although Australia was the home of the most ancient songbirds,’ they argue, ‘Australia itself appears to have played a marginal role in the dispersal to other continents.’ They believe a small number of Australian songbirds – possibly just a few individuals or a single flock – flew out to sea around 33 million years ago and made their home on platforms of dead and crumbling coral. Those platforms were ‘proto-islands’ which, over tens of millions of years, combined to become New Guinea. Eventually mountain ranges carved its dense forests into pockets where isolated populations rapidly diversified. Still, New Guinea was considered an unlikely source of the songbird radiation: ‘Islands were for a long time believed to be dead ends in evolution.’

Animals that had adapted to island conditions were long assumed to be disadvantaged on the mainland – they often lost their fear of predators, for example. Passerine phylogeny offers a more complex story. ‘Islands seem often to be colonised by dispersive, adaptive and smart, social and pair-breeding birds,’ the authors write. The songbird lineage split into two cohorts – Corvides and Passerides – probably around the time that the ancestor discovered the proto-islands of New Guinea. Both lineages diversified as New Guinea grew, with many of the Corvides adapting to the mountain forests, including the birds-of-paradise. The nearly eight hundred species of Corvides are still most densely distributed on New Guinea. But a few early species in both cohorts travelled further, to new volcanic islands in nearby seas.

Songbirds dispersed as many as fifteen times from New Guinea into Indonesia, Asia and possibly as far as Africa. They went ten times into Polynesia and returned several times to Australia too. Around thirty million years ago, the authors estimate, a single species of Passerides – perhaps related to Australian robins – reached mainland Asia, probably by using islands as stepping stones, and diversified quickly across the northern hemisphere. Major lineages such as tits, wrens and finches emerged in the Himalayas and other Asian mountains, while the radiation of larks, swallows and Old World warblers got started on the African continent. Small passerine bones rarely fossilise, but palaeontologists have found traces of tit, kinglet and waxwing-like birds in Europe from 25 million years ago. Some 3800 songbird species descended from the first Passerides to reach mainland Asia, including four of the five passerine families with worldwide distributions today.

Songbirds’ global radiation was probably accelerated by a fluctuating climate, as they searched for breeding grounds and food. Unlike the many Corvides that mate for life, Passerides usually pair up only for a short breeding season. Many Passerides are fully migratory, which suggests their ancestors were as well. These birds could have spread into northern Asia and Europe in warmer months while at the same time budding off populations into regions along their migratory routes that could sustain residents year-round.

Song itself helped songbirds diversify, as populations developed unique calls that distinguished them from closely related neighbours. No one knows exactly when songbirds learned to sing. It is usually suggested that song originated with the Australian passerines. But it is also possible that passerines and parrots inherited vocal learning from their common ancestor, and that the suboscine and New Zealand lineages somehow lost the ability.

By the start of the Miocene epoch, 23 million years ago, the world was populated by ancestral representatives from most of the 236 taxonomic families now used to classify songbirds. Around this time, Corvides began dispersing from New Guinea. Most remained non-migratory and tropical. The ancestors of crows and jays, however, evolved bigger bodies, to conserve heat, and strategies for feeding in winter, such as hoarding and scavenging. They radiated to become the fifth passerine family with worldwide distribution. Ravens are the largest songbirds on earth today.

During the Miocene, grasslands replaced forests around the globe. This was a boon for the finch-billed songbirds, which could crack grain seeds with their powerful beaks and digest gluten and starch. One ancestral finch-billed songbird species crossed to North America (possibly over the Beringian land bridge) around fifteen million years ago. The most colourful and diverse assemblage of songbirds are descended from it: the 850 different New World sparrows, New World warblers, New World blackbirds, cardinals, grosbeaks (above) and tanagers. These songbirds and others, especially wrens, eventually met their long-lost relatives in South America. Passerine diversity is highest today in regions of the Andes and Amazon where oscine and suboscine birds coexist.

One of the last places the songbird radiation reached was another chain of islands. Recent genetic tests show that Galápagos finches are speciating at a faster rate than any other group of birds. Darwin didn’t give much thought to them until he returned to London in 1836 and started to observe the pattern of ‘descent with modification’ in their varied beaks. He was still working privately on his theory more than twenty years later when he received a letter from Indonesia. It was Wallace, on his way to New Guinea, laying out many of the same ideas. The songbird radiation links the islands – ten thousand miles apart – that inspired the two naturalists who separately came up with the first working theory of evolution.