ORNITHOLOGICAL MASTERCLASS 2 Bird Evolution Chris Cooney and Gavin Thomas take us on a whistle-stop tour through ▲ Fossils like Archaeopteryx have the last 165 million years to explain just where today’s birds come from. revolutionised our understanding of avian evolution. How did birds evolve? Over the course of 150 million years, and vaned feathers – with a stiff central Neornithes) did not become diverse and birds have evolved from a single common shaft and hooked hair-like barbules – just widespread until after the end-Cretaceous ancestor to the 10,000 species alive today. like modern birds. (K-Pg) mass extinction. Recent work But what is the evolutionary origin of this We now know that birds belong to a using genomic data has revealed diversity? This remained one of the most group of dinosaurs called the theropods, two remarkable (and controversial) fiercely debated questions in evolutionary which includes the giant Tyrannosaurus insights. First, the majority of bird biology for decades and only reached a and the diminutive Velociraptor (famous orders originated after the K-Pg mass scientific consensus in the 1990’s – not all for their oversized appearance in the first extinction. Second, the early branches of dinosaurs were wiped out during the mass Jurassic Park film). Vaned feathers appear the Neornithe tree of life may not have extinction at the end of the Cretaceous in the theropod group Maniraptora and followed a ‘bifurcating’ pattern where period, 65 million years ago. Some wings in the Paraves (Fig 1). Exactly each branch splits into two. Instead, the survived, and these are birds. which fossil represents the earliest bird is early expansion of non-passerine bird The discovery of Archaeopteryx in still debated but current evidence suggests orders was an explosion of evolution, with Germany in 1861 provided the first major that Archaeopteryx was a true bird and up to eight major groups emerging in the clue. Archaeopteryx had bird-like features, the earliest known representative of the geological blink of an eye. The reasons such as wings and feathers, alongside taxonomic class Aves, dating the origin for this explosive expansion are uncertain other distinctly non-avian features; a bony of birds to the mid-late Jurassic period but their emergence in a rapidly changing tail, forelimb claws (seen today only in (165‒150 million years ago). world following widespread extinction juvenile Hoatzin) and teeth. In the late- Numerous bird groups appeared may have allowed birds to exploit and 1990s thousands of fossils from the early and went extinct in the Jurassic and adapt to previously inaccessible resources. Cretaceous (130‒120 million years ago) Cretaceous periods. Two groups, the Over the next 65 million years, birds were discovered in north-eastern China, toothed, flying Enantiornithines and became the most successful group of providing compelling evidence of the the large, mostly flightless, diving terrestrial vertebrates, spreading across transition from dinosaurs to birds. These Hesperornithiformes, lived throughout the globe and becoming more species rich PHOTOGRAPHS: BLICKWINKEL/ALAMY, MARK CARWARDINE/NATUREPL.COM MARK BLICKWINKEL/ALAMY, PHOTOGRAPHS: HAWTIN (NIGELHAWTIN.COM) INFOGRAPHICS: NIGEL remarkably complete fossils have wings the Cretaceous. Modern birds (the than mammals, amphibians and reptiles. 16 BTO NEWS | Winter 2016 Ornithological masterclass EVOLUTION Rate of diversification Low High Ostriches and kiwis Water and landfowl Ornithischians Hummingbirds, Sauropods swifts and nightjars Tyrannosaurus Cuckoos, bustards and pigeons Oviraptorosauria Cranes Theropoda Dromaeosauridae Diving, wading Troodontidae Maniraptora and shorebirds Archaeopteryx Paraves Hawks Jeholornis Owls Confuciusornithiformes Aves Kingfishers, Sapeornis bee-eaters and rollers Enantiornithes Falcons Parrots Vorona Hesperornithiformes Ichthyornis Neornithes Songbirds, including Triassic tits, crows, thrushes, Jurassic Neogene Quaternary Palaeogene Cretaceous flycatchers and finches 250 200 150 100 50 0 Million years ago FIG 1: THE TREE OF LIFE OF BIRDS Pittas, antbirds, Birds evolved from theropod dinosaurs and ovenbirds and tyrants the number of species expanded rapidly after the K-Pg mass extinction. The Neornithes tree shows the diversity of major groups (relative size Cretaceous Palaeogene Neogene Quaternary 80 60 40 20 0 of triangles) and those groups that form new Million years ago species fastest (dark red colours). Conserving bird evolutionary diversity The evolutionary tree of life contains a wealth of information beyond the relationships among species. For instance, we can identify which species have few close relatives and evolved long ago. If these species became extinct, we would lose a disproportionately large amount of evolutionary history that can never be recovered. Some researchers have suggested that preserving species that represent unique parts of the tree is critical. Sadly, with limited funding it is often necessary to make pragmatic decisions about which species to prioritise for conservation action. By putting the most at-risk species first, and then ranking species according to the amount of evolutionary history that they represent, we can identify species that are on the ‘EDGE’ (Evolutionary Distinct, Globally Endangered). The top five ranked species are as diverse as they are unique: Giant Ibis, New Caledonian Owlet-nightjar, California Condor, Kakapo and Kagu. Until recently, the idea of ▲ New Zealand's combining evolutionary history with threat status was largely critically endangered confined to the pages of academic journals, but we are finally Kakapo had a known seeing real-world applications, most notably through the adult population of 123 Zoological Society of London’s EDGE of Existence initiative. individuals in June 2016. BTO NEWS | Winter 2016 17 The global diversity of birds Today, the diversity of birds is far from the distribution of birds globally. Around Wrens: adaptation equally distributed. Some groups have 250 species breed in Britain, an area of expanded spectacularly, like the passerines 244,000 km2. This compares to roughly in action with almost 6,000 species, whereas 1,500 species in Ecuador (284,000 km2). others, like the Hoatzin and Oilbird Species numbers peak in the Tropics, but Data from ringing and the Breeding Bird are evolutionary oddities with few close what causes this variation? Since birds Survey show how tiny Wrens are locally relatives. A long-standing question is first evolved, the Earth’s tectonic plates adapted to the differences in the Great whether species rich groups are older have moved and continental landmasses British climate. Wrens in Scotland are than groups that are species poor. The have shifted, but the regions that form larger and can store more body fat than underlying idea is simple: all else being the Tropics today have been climatically those further south, making them better equal, a group that arose 100 million stable, while temperate and Polar regions able to withstand cold weather. Whether years ago should contain more speices have experienced many changes. This has Wrens’ rate of evolutionary adaptation is than one that arose only 10 million years led to the suggestion that species have able to keep pace with the rate of climate ago. However, the evidence points to an steadily accumulated in the Tropics over change remains to be seen. alternative explanation – some groups are millions of years – species originate and www.bto.org/wrens-adapt predisposed to give rise to new species themselves give rise to new species with at faster rates than others. The diversity little impact of climate-driven extinctions, of different major bird groups is closely whereas similar species accumulation in related to the rate at which new species temperate and Polar regions could be u Wren body mass varied originate. In some groups, new species tempered by extinction due to climatic by around 5% form at remarkably high rates – like volatility. It has also been suggested that between the songbirds – whereas in others the rates speciation rates are higher in the Tropics, warmest and are slow. Much current research is focused but for birds there is little evidence of this. coldest regions. on measuring how fast species arise (the In fact, speciation rates seem to be highest speciation rate) in an effort to tease apart on average across much of the New World why this varies so much. (North and South America) and on many Just as the distribution of species in islands, but lower in the Old World PHOTOGRAPHS: ALLAN DREWITT/BTO PHOTOGRAPHS: major bird groups is uneven, so too is (Europe, Africa, Asia). FIG 2: GLOBAL MAP OF THE DIVERSITY OF THE WORLD’S BIRDS Species per 10 km2 Species richness peaks in the Amazon forest of South America and in tropical mountain ranges. Insufficient 100 200 300 400 500 data 500 400 300 Number of species 200 100 Source: Based on distribution data from BirdLife International and NatureServe 2015. 18 BTO NEWS | Winter 2016 Ornithological masterclass EVOLUTION How do bird species multiply? A species is defined as populations whose members and adapting to their respective habitats. Over time, The authors can interbreed with one another. So how does changes in behaviour and appearance accumulate to Chris Cooney is a one species split into two? Islands provide natural the point where individuals from each population postdoctoral researcher at experiments – ‘laboratories of evolution’ – that offer do not mate if they come back into contact.